Gems!

To carry on from Albert’s article on diamond, cubic diamond crystals in from left to right: beach conglomerate from Namaqualand (left), kimberlite from Siberia (bottom) and kimberlite from South Africa (right). (Gemstones, Geological Museum)

To carry on from Albert’s article on diamond, cubic diamond crystals in from left to right: beach conglomerate from Namaqualand (left), kimberlite from Siberia (bottom) and kimberlite from South Africa (right). (Gemstones, Geological Museum)

Like all other original branches of science, geology has split and evolved into a multitude of sub-species such as geophysics, petrology, mineralogy, volcanology and seismology. The science that deals with natural and artificial gemstones, gemology, is considered to be part of the geosciences and specifically a branch of mineralogy. If we disregard artificially generated radioisotopes, certain gems are by far the most valuable items as measured in relation to size and weight. They can be and are considered desirable on account of their intrinsic beauty, the social status conferred upon their wearer, their rarity or scarcity, their collectability and their monetary value. Some specimens are even used as a non- or low tax substitutes for cash in the same manner as the super-rich invest in art or football teams.

Egyptian faience floral collar containing polished beads, cabochons and inlays of carnelian, lapis lazuli, turquoise and also a scarab made from Libyan desert glass of meteoric origin. (uncertain provenance)

Egyptian faience floral collar containing polished beads, cabochons and inlays of carnelian, lapis lazuli, turquoise and also a scarab made from Libyan desert glass of meteoric origin. (uncertain provenance)

Mankind has worn jewellery for at least 20,000 years as finds of necklaces and other ornamentations made from shells, ivory and bone, worked or unworked, in Neolithic graves testify. Egyptian tombs have contained gemstones beautifully carved in the semblance of deities or animals, predominantly in the form of the holy dung beetle known as scarabs, as well as exquisite pieces of gold jewellery. The Romans made extensive use of jewellery and gems, not only for personal adornment but as signet rings used to seal documents. They even carved vessels from agate, garnet, beryl and other large crystals. Originally reserved for the wealthy and powerful as a visible means of demonstrating both their affluence and influence, the use of gems for personal adornment has today expanded to the point where almost every human being owns a piece of jewellery, or at least their wives do.

In antiquity, gems were differentiated between and given names, usually in Latin or Greek, based on their colour, their origin or their superstitiously perceived properties such as Amethyst (Greek “amethystos” – not inebriated) which was thought to protect its wearers from overindulgence. As an example of the first, carbunculus (lat. glowing coal or ember) was indiscriminately applied to all deep red gems; ruby, spinel and garnets. This is why nearly all the truly large, historical red gems such as the “Black Prince’s Ruby” in the Imperial crown on display in the Tower, are called “rubies” when in fact they are spinels. Great quantities of mostly darker, purplish-hued carbuncles were obtained in trade at the town of Alabanda (modern Turkey) and when we acquired the technology to differentiate between mineral species in the 19th Century, this in turn gave the name of the most common garnet species; Almandite, the iron-aluminium variety of garnet – Fe3 Al2 (SiO4)3.

Intaglio or engraved garnet carrying the image of the Emperor Hadrian (suebrownjewels)

Intaglio or engraved garnet carrying the image of the Emperor Hadrian (suebrownjewels)

One of the first writers on gemmology was the Roman naturalist Caius Plinius Secundus, Pliny the Elder, who perished during the 79 AD eruption of Vesuvius. At that time, carbuncles were divided into Indian and Garamatic (Carthaginian) after their trade origin. Pliny further subdivided the “Carthaginian anthrax gems” of Theophrastos into two classes; male and female, where the former, considered more valuable, were darker and purplish in shadow but appeared as flame-red in transmitted sunlight, something that together with his information that they were used in signet rings identifies the gems as almost certainly being garnets.

Another curious example is that of “topaz”. Most modern people will associate topaz with a gem of predominantly yellow colour but originally, both name and colour referred to a different gemstone. The name is derived from the Red Sea volcanic island of Zebirget or St John’s which during antiquity was known as Topazos. The gems mined there were green in colour, a green not uncommonly with a yellowish to brownish tint. Today, we know that the gems mined at Zebirget were peridots; gem quality specimens of the mineral olivine, a mixture of the end members fayalite and forsterite, as mentioned in Albert’s article on diamonds.

But with desirability also comes greed and no other emerging science has been so preposterously, repeatedly and continuously embattled by greedy individuals as gemology. One of the first scams perpetrated was misrepresentation in order to increase desirability and extract a higher price. Specimens of lowly smoky quartz are still flogged as “smoky topaz”, something they certainly are not. Garnets, a far less valuable gemstone than rubies, were marketed as Cape-, Arizona-, Brazil, Ceylon- (etc) “rubies”. Rock crystal was offered as Bohemian, Arkansas, Marmarosch or Alaska “diamonds”, soapstone as Korean “jade”, various green gems as [locality +] “emeralds”. Sometimes, the mistake is genuine as with the find of doubly terminated quartz crystals in Herkimer County, N.Y. state, which were mistaken for diamonds and presented as such in an attempt to pay for French arms to be used in the War of Independence (oral tradition). To this day, these crystals are marketed as “Herkimer Diamonds”.

As their appearance may hint at, it is easy for the inexperienced to confuse the doubly terminated quartz crystal with only a short, blocky crystal prism (the pointy ends are called pyramids) for diamond and the dolomite matrix, “mother rock”, with kimberlite. The so-called “Herkimer Diamond” is a collectible in its own right as not only are the euhedral (well-formed), doubly-terminated crystals aesthetically pleasing. They often contain interesting and readily visible two-phase inclusions consisting of water and gas bubbles. (WikiMedia Commons)

As their appearance may hint at, it is easy for the inexperienced to confuse the doubly terminated quartz crystal with only a short, blocky crystal prism (the pointy ends are called pyramids) for diamond and the dolomite matrix, “mother rock”, with kimberlite. The so-called “Herkimer Diamond” is a collectible in its own right as not only are the euhedral (well-formed), doubly-terminated crystals aesthetically pleasing. They often contain interesting and readily visible two-phase inclusions consisting of water and gas bubbles. (WikiMedia Commons)

Another subterfuge used is to name a find of strikingly coloured gems of an already known species and pretend that only gems from that locality are entitled to the name, just like “champagne” can only be used for sparkling wines from the French province of Champagne. Examples of this are the rose-coloured pyrope-almandine garnets first found in Macon County, North Carolina, in the late 19th Century and marketed as Rhodolite and the vividly coloured blue to bluish-green tourmalines found at the Paraiba mine, Brazil, in the late 1980s which is marketed as “Paraiba Tourmalines”. Of course, humanity being what it is, it wasn’t long before tourmalines of indifferent colour were marketed as “Paraiba” in order to increase desirability and to extract a higher price for sub-standard wares.

The Properties of Gemstones

Gems are commonly divided into three main groups; diamonds, coloured gems and substitutes (artificial or simulations). This and different subdivisions are based on the intrinsic and extrinsic properties of the various gem species. The intrinsic properties, those that derive from the physical properties of the gem material from which they are cut or shaped, are colour, clarity, hardness, cleavage, fracture and optical properties such as refractive index, dispersion, double refraction, asterism, colour play, opalescence, iridescence, chatoyance and adularescence. The extrinsic properties are cut, rarity, desirability, collectability and monetary “value”.

Colour. Gems can either be idiochromatic or allochromatic. In the former case, one or more of the chemical constituents of the crystal itself lends the gem a specific colour such as the red of almandite garnet, the green of malachite, the dark blue of lapis lazuli or turquoise blue of the turquoise. Most gems are however allochromatic. They are perfectly colourless if pure and derive their colour from impurities such as the red of ruby and blue of sapphire (mineral: corundum) or the green of emerald and sea-blue of aquamarine (mineral: beryl). Some allochromatic gems derive their colour from defects in the crystal lattice such as diamond or smoky quartz. These defects can be induced by either minute substitutions, 1/1,000,000 carbon atoms being replaced by boron in the case of blue diamonds, or natural radiation such as with smoky quartz.

The allochromatic colours of corundum (Al2O3): Ruby – red, Sapphire – blue, all others are designated [colour prefix] + sapphire, eg. pink sapphire, yellow sapphire, colourless sapphire, green sapphire. While pure corundum is colourless, the red colour comes from minute amounts of chromium and the blue from a titanium-iron biparticle, which replace some of the aluminium. Even if the colour of pink sapphire comes from chromium, the colour is not saturated enough to merit the designation ruby. (Gemstones, Geological Museum)

The allochromatic colours of corundum (Al2O3): Ruby – red, Sapphire – blue, all others are designated [colour prefix] + sapphire, eg. pink sapphire, yellow sapphire, colourless sapphire, green sapphire. While pure corundum is colourless, the red colour comes from minute amounts of chromium and the blue from a titanium-iron biparticle, which replace some of the aluminium. Even if the colour of pink sapphire comes from chromium, the colour is not saturated enough to merit the designation ruby. (Gemstones, Geological Museum)

Colour can be enhanced or changed by heating or irradiation. The best known example of the former is when amethyst is heated to ~400C where it changes colour to yellow and is then sold as citrine. That said, even if the majority are the result of heating amethyst there are naturally occurring citrines. The best example of irradiation being used to induce colour is in topaz. Naturally occurring topaz is mostly colourless, but if subjected to irradiation, this changes to blue. Almost all blue topaz on the market is irradiated and with the deeper shades, usually marketed as “London blue” or “Swiss blue”, the stones may retain harmful levels of radioactivity. If you are collecting minerals, beware of opaquely black smoky quartz as they are sometimes obtained by hard irradiation of colourless quartz crystals; rock quartz, the rationale behind being that rock crystal is so abundant that you can multiply your profit by turning it artificially to smoky quartz specimens.

gems P4 diamond

This “brilliant” cut diamond has been given the lowest grade for clarity I3/P3 (Included or Piqué). Not only is it downright ugly, there are also durability issues with such a stone as the inclusions cause serious weaknesses in the crystal lattices. This is an industrial grade diamond and it should never have been cut. (tradeindia)

Clarity is of paramount importance when it comes to brilliant cut diamonds as any internal imperfection will lower the yield of reflected light from the gem. There is a premium on perfectly clear or almost clear diamonds, If the inclusions are large enough to be visible to the naked eye, it severely diminishes the appeal of the gem and hence its price. It is still important with coloured gemstones where a gem appearing clear to the eye is more desirable whereas unsightly, large inclusions will invariably lower the desirability and hence the value of the gem. With star rubies/sapphires (asterism), cat’s or tiger’s eye gems (chatoyance) and moonstone (adularescence) the reverse is to a certain extent true as the root cause of the desired optical effects derive from inclusions in the gemstone.

Hardness, cleavage and fracture. All these intrinsic properties affect the durability of the gemstone. Hardness is usually measured on the Mohs Scale which ranges from 1 (talcum) to 10 (diamond) where each successive mineral has the ability to scratch the preceding ones. As quartz dust with a hardness of 7 is omnipresent, gems cut from minerals with a lower hardness will be scratched and worn surfaces will result. Cleavage is the result of the mineral cohesion/bonding being substantially weaker along one or more planes of the crystal such as with diamond and topaz. Fracture. Some minerals while being both hard and not suffering from cleavage may still be prone to fracturing, the prime example being emerald. Please remember that rings tend to take both hard knocks and are subject to greater wear than earrings or pendants, so try to avoid ones set with softer gems, especially those subject to cleavage or fracture, or in the case of emerald ones, take due care! You don’t have to worry about the diamond engagement ring though as the cleavage of diamonds takes a lot of precisely directed force if it is to occur . In a way it is a good thing as it allows the splitting of large rough diamonds for ease of cutting with minimal loss.

The 58-facet brilliant cut. The top part of the gem is known as the crown and consists of table, eight crown star facets, eight crown main facets and sixteen crown girdle facets. The bottom part is referred to as the pavilion and consists of sixteen pavilion girdle facets, eight pavilion main facets plus in diamonds usually a tiny flat facet at the bottom to protect the gem from cleavage known as the culet. The percentages are in reference to the maximum width of the gem as measured across the girdle (which separates crown from pavilion) and the angles are those of the crown and pavilion main facets. These are the proportions of the “Tolkowsky ideal brilliant cut” for diamonds. (Adapted from Gemstones, Geological Museum)

The 58-facet brilliant cut. The top part of the gem is known as the crown and consists of table, eight crown star facets, eight crown main facets and sixteen crown girdle facets. The bottom part is referred to as the pavilion and consists of sixteen pavilion girdle facets, eight pavilion main facets plus in diamonds usually a tiny flat facet at the bottom to protect the gem from cleavage known as the culet. The percentages are in reference to the maximum width of the gem as measured across the girdle (which separates crown from pavilion) and the angles are those of the crown and pavilion main facets. These are the proportions of the “Tolkowsky ideal brilliant cut” for diamonds. (Adapted from Gemstones, Geological Museum)

Refractive index. Every mineral has an associated refractive index, how much it bends incoming light. In optics, we talk about something called the critical angle which is the angle at which light no longer passes into the transparent object but is reflected from its surface. This is vital to facet cut gemstones as the yield of light reflected back to the viewer depends on the critical angle. The higher the refractive index, the more light is returned to the eye and the more brilliant the gem is perceived to be. In facet cut gemstones the bottom-most facets, the so-called pavilion main facets, are supposed to act like mirrors. If they are cut at too shallow an angle so that it does not exceed the critical angle of the mineral, no light will be returned to the eye. The greater the difference between the angles of the pavilion main facets and the critical angle of the gem, the more light is returned to the eye and this is why diamonds appear so brilliant.

Dispersion. This is a measurement of the difference in refraction between read and blue light of a given material and is what gives a gem its fire, or rainbow of colours. Of natural gemstones, diamond has one of the highest figures for dispersion with 0.044 whereas quartz has one of the lowest at 0.013 which means that diamond has more than three times the fire of quartz. In order to be readily visible, the gem should ideally be colourless as a strong body colour will mask the fire such as happens with the green demantoid garnet, a variety of andradite garnet, which has a figure for dispersion in excess of diamond at 0.057.

Brilliant cut demantoid garnet, diamond and colourless zircon. While the demantoid garnet has greater dispersion or “fire” than diamond, this is masked by the strong body colour. The double refraction of zircon is readily apparent as a doubling of the edges of the pavilion facets at the bottom of the gem.

Brilliant cut demantoid garnet, diamond and colourless zircon. While the demantoid garnet has greater dispersion or “fire” than diamond, this is masked by the strong body colour. The double refraction of zircon is readily apparent as a doubling of the edges of the pavilion facets at the bottom of the gem.

Double refraction. As gems are minerals, they belong to different systems of crystal symmetry. Only gems that belong to the cubic system (diamond, spinel, garnet) where each of the three crystal axes are identical are singly refractive. All other gems exhibit double refraction to some degree, the most famous being “Iceland spar”, calcite. As zircon (not to be confused with the synthetic cubic zirconia) is highly doubly refractive, something that is readily apparent even under moderate magnification such as that of a magnifying glass, every discerning young woman could easily tell if she had been given a diamond or a zircon engagement ring. (The artificial Cubic Zirconia should not be confused with zircon!)

From top left to bottom right: The asterism of this 23 mm star ruby is caused by light reflected off tiny needles of rutile aligned according to the trigonal nature of corundum. The chatoyance of this 7½ by 8 mm chrysoberyl catseye gem is due to light reflecting off parallel needles or microscopic tubes. In order to show the cateye-effect, the gem has to be cut in such a way that the needles are parallel to the base of the gem. The colour play of opal is caused by light reflecting off the microscipic globules of silica, the perceived colour being dependent on the size of the individual globules. The adularescense of moonstone appears as a soft white to bluish glow throughout the gem and is caused by light reflecting of minute lamellar structures. A particularly appealing form of the feldspar mineral labradorite was discovered in Finland and given the trade name spectrolite. The vivid colours are due to light reflections from thin, lamellar structures. Sometimes the inclusions themselves can be what makes a gem interesting as this quartz gem with golden rutile needles arranged in a star.

From top left to bottom right: The asterism of this 23 mm star ruby is caused by light reflected off tiny needles of rutile aligned according to the trigonal nature of corundum. The chatoyance of this 7½ by 8 mm chrysoberyl catseye gem is due to light reflecting off parallel needles or microscopic tubes. In order to show the cateye-effect, the gem has to be cut in such a way that the needles are parallel to the base of the gem. The colour play of opal is caused by light reflecting off the microscipic globules of silica, the perceived colour being dependent on the size of the individual globules. The adularescense of moonstone appears as a soft white to bluish glow throughout the gem and is caused by light reflecting of minute lamellar structures. A particularly appealing form of the feldspar mineral labradorite was discovered in Finland and given the trade name spectrolite. The vivid colours are due to light reflections from thin, lamellar structures. Sometimes the inclusions themselves can be what makes a gem interesting as this quartz gem with golden rutile needles arranged in a star.

Where do gems come from?

Gems are formed under very specific conditions of high pressure and temperatures in the crust or mantle of the Earth. The major processes were described in an earlier Volcanocafé article, Mineralisation Processes . For each mineral, there is a solidification point at which it crystallises out of the melt and in order for a perfect or near perfect crystal to form, one large and pure enough to be cut into a gem, conditions will have to have been stable for a very long time, in some cases many thousands of years.

Furthermore, gems form at different stages and may even form in stages. The first ones to form and do so in the mantle are pyrope garnets, diamonds and the silica tetrahedrons (cristobalite) necessary for the later formation of most silicate minerals and gems such as quartz, beryl, tourmaline, topaz and the majority of the garnets which are formed in the crust during the mineralization stages when magma cools, differentiates and fractionates. Other gems are formed as a result of weathering and subsequent mineralisation after sediments are formed such as turquoise and opal. Yet others form as sediments come in contact with magma and metamorph such as corundum (ruby and sapphire), spinel and certain garnets. Some form as a result of the high pressures encountered during subduction such as jade.

The origin of the most common gem minerals: 1) Turquoise forms from the decomposition of copper-bearing volcanic rocks under pressure, 2) Opal forms in sediments due to the evaporation of silica-rich groundwater (this is why some opal is formed from fossils), 3) Beryl (aquamarine), quartz, tourmaline, topaz, chrysoberyl, spessartite garnet, moonstone form in pegmatites near or in granitic plutons, 4) Impure shales and limestones that have come in contact with hot pegmatitic or pneumatolytic fluids metamorphoses into 4a) ruby, sapphire, spinel and zircon (high temperature), 4b) lapis lazuli, 4c) spessartite and grossularite garnet (skarn), 5) If the pegmatitic/pneumatolytic fluids encounter chromium bearing rocks, emeralds form, 6) Ruby, sapphire, spinel, chrysoberyl and garnets forms by heat-induced metamorphosis of aluminium-rich sediments, 7) Peridots form in basalt, 8) Jadeite is formed by the high pressures of subduction (to the lower right), 9) Diamond, pyrope garnet and silica tetrahedrons form in the mantle, 10) Gem minerals formed at depth and forming during the ascent of magma, 11) As the magma reaches the surface, these gem minerals are deposited, 12) As the magma/lava cools, quartz minerals (rock crystal, smoky quartz, amethyst, citrine and some opal) are deposited in gas-filled cavities from silica-rich fluid remnants, 13) Due to uplift, gem-bearing rocks are brought to the surface, 14) weathering/erosion releses the gems into the soil, 15) Rivers transport the gem-bearing soils and deposits them in gravel beds, so-called alluvial deposits, 16) similarly, jadeite boulders are accumulated in rivers. (Gemstones, Geological Museum)

The origin of the most common gem minerals: 1) Turquoise forms from the decomposition of copper-bearing volcanic rocks under pressure, 2) Opal forms in sediments due to the evaporation of silica-rich groundwater (this is why some opal is formed from fossils), 3) Beryl (aquamarine), quartz, tourmaline, topaz, chrysoberyl, spessartite garnet, moonstone form in pegmatites near or in granitic plutons, 4) Impure shales and limestones that have come in contact with hot pegmatitic or pneumatolytic fluids metamorphoses into 4a) ruby, sapphire, spinel and zircon (high temperature), 4b) lapis lazuli, 4c) spessartite and grossularite garnet (skarn), 5) If the pegmatitic/pneumatolytic fluids encounter chromium bearing rocks, emeralds form, 6) Ruby, sapphire, spinel, chrysoberyl and garnets forms by heat-induced metamorphosis of aluminium-rich sediments, 7) Peridots form in basalt, 8) Jadeite is formed by the high pressures of subduction (to the lower right), 9) Diamond, pyrope garnet and silica tetrahedrons form in the mantle, 10) Gem minerals formed at depth and forming during the ascent of magma, 11) As the magma reaches the surface, these gem minerals are deposited, 12) As the magma/lava cools, quartz minerals (rock crystal, smoky quartz, amethyst, citrine and some opal) are deposited in gas-filled cavities from silica-rich fluid remnants, 13) Due to uplift, gem-bearing rocks are brought to the surface, 14) weathering/erosion releses the gems into the soil, 15) Rivers transport the gem-bearing soils and deposits them in gravel beds, so-called alluvial deposits, 16) similarly, jadeite boulders are accumulated in rivers. (Gemstones, Geological Museum)

Once the gem-bearing rocks have been brought to the surface by a combination of uplift and weathering, the gems contained become accessible to mankind. They can either be mined directly from the rocks in which they formed or from alluvial deposits where erosion has already done the mining for us, more gently and also destroying the imperfect specimens. This is why alluvial deposits are favoured by those mining for gems and why rockhounds search out the pegmatites and other rock formations where they can find intact crystal specimens. But irrespective if we are fascinated by volcanoes, the amazing symmetry of crystals or the beauty of cut gems, they are all aspects of our fantastic Planet Earth.

Henrik

54 thoughts on “Gems!

  1. I love to read about these volcano related beauties (although personally I pefer the cryptocrystalline forms of SiO 😉 ).
    Thanks Henrik for the nice article.

    • Such as that lovely slice of (untreated!) agate? There are indeed infinite varieties to wonder at and gawk over. 🙂

    • Far beyond fictional… but appropriate. Bringing it back to geology… Most of Port Royal sits at the bottom of Kingston Bay due to wide scale liquefaction of the sand spit that it sat on → Mw 7.5 on June 7, 1692 Somewhere around Jamaica, the Enriquillo-Plantain Garden Fault Zone becomes the Walton Fault zone and forms the southern boundary of the Gonâve microplate. This is the same fault zone that yeilded the 2010 Haitian quake.

      On a more serious note, Andrew Jackson had the assistance of Jean Lafitte during the battle of New Orleans. And I am quite sure that the cypress swamps, snakes and gators had a serious impact on the ability of the British to maneuver. River to one side, swamps to the other. I don’t recall where, but at the time, I think Britain was tied up in other engagements around the world and did not mount the resources needed to break the defenses. Hindsight is 20/20, and I’m pretty sure that they would have eventually succeeded had they put their mind to it.

      https://youtu.be/_xR0h6FGCBY

    • A bit closer maybe? The crater is quite wide and those steam emissions look like they may be from more of it?

    • Basically… a potential black swan just got a few more feathers.

      • Does the population as a whole understand just what this means? I think not!

        • As a whole? No, not at all. The only thin on their minds is where to score the next fix or to get laid. The more sane think about the next business deal or if they can sell something. A very few, like TGMCCOY actually have a clue and do preparatory things like opting to not live in the higher hazard areas.

          Remember, San Fransisco is where some people are leasing out basement crawl spaces for $500 /mo.

  2. Bardy looks like having another magama Chamber or same one filling up again… maybe another eruption somewhere along the line of time or movming magma! Even the IMO has made note of it this morning.
    Quote IMO “An earthquake of magnitude 3.0 occurred in the northern rim of the Bárðarbunga caldera at 01:24 this night.”

    http://en.vedur.is/earthquakes-and-volcanism/earthquakes

    Maybe something to see here so don’t move along just yet!

  3. Henrik. Thank you for your article on Gems :)… I missed out on the diamond I hoped to receive after an earlier post.So not holding out much hope for free samples after this one either .

    My favourite gemstone is Emerald, Be3Al2SiO6 A deep intensly green form of Beryl. A brittle stone, unlike diamonds ,it is brittle so very prone to crack and so flawless or almost flawless intensely green emeralds, rare and incredibly valuable or more likely it is laboratory made, or if natural it is light coloured beryl treated with oil and or heat. You are not likely to get a really good Emerald on eBay, Always best to go to a reputable jewelers Beware though of some of the big chains of jewelry stores or catalogs…
    Remember Mr Ratner ? he owned a chain of popular Jewelry stores in the UK.
    His life fell apart in 1991 when he gave a speech to an audience of 5,000 at an Institute of Directors conference. During this, he notoriously described his own company’s products as ‘crap’. The speech went down in business history and Ratner’s career went down the pan. He spent 18 months trying to limit the damage, but was then fired from his own company. Even today he is remembered as every time someone makes a huge mistake in business or commerce it is referred to as “Doing a Ratner”

    • I didn’t know that !
      I actually worked for Ratner’s briefly..about 6 months just a year or two before that, at the H. Samuel warehouse in Hockley, Birmingham UK. I was temping in their accounts office, and even from there it was interesting (after a fashion…up to a point) as I was logging despatch notes. I noticed that they were actually able to make money by shipping finished goods to the Channel Islands, where they had warehouses but no outlets, then shipping the goods back to mainland UK some time later. I forget the exact why and wherefore of it, but it was something to do with tax laws at the time.
      I now own a shop which some describe as a “New Age” shop… not being a believer in the vast majority of that particular family of paradigms I’m not quite sure what to make of that !
      But I do sell crystals and minerals, and I have to say, largely as a compromise with my wife. I personally, much as I love them, wouldn’t sell them because I find it (the new age crystals market) to be cynical and exploitative for exactly some of the reasons mentioned in this most excellent article…..

      Although… Herkimer diamonds… really are things of great natural beauty.There’s far more beauty and mystery in the science of the creation of such things than many who wear them could possibly imagine. And certainly no need to place extra meaning on them. They have quite enough of that without any help.

      Great article, Henrik, Thank you.

      • Sometimes on local car boot sales (yard sales for the Americans here) I have seen ‘New Age’ types selling their mystic crystals. Almost gives me the inclination to get out my old geologist’s hammer and go prospecting. Almost.

    • You are absolutely correct about how much emerald is treated, “prettified”, and your advice to only buy one from a reputable dealer very sound (albeit one certified by a professional gemologist belonging to either the GIA or the Gemmological Association of Great Britain is even better).

      May I suggest the vanadium grossular garnet “tsavorite” instead? It is a far superior gem to emerald – it is not brittle, it is harder than quartz (albeit slightly softer than beryl), it has a substantially higher refractive index (1.74 against 1.58) which makes it appear more lustrous. It is expensive (desirability + scarcity, especially in sizes greater than ~2.0 ct), but not as expensive as a top grade, GIA-certified emerald from a reputable dealer is.

    • The cool thing about emeralds is that you need special circumstances to create the colour. Beryllium is common enough in pegmatites, but the chromium (or vanadium if you´are not a purist) is usually found in ultramafic rock. So basically you need a way to inject fluids containing the leftover light elements from granite formation into a rock containing the needed coloring agents. Colombian emeralds get their chromium from a black schist if I recall correctly and have a nice vivid green. The Brazilian emeralds I am more familiar with are from pegmatitic intrusions into large older ultramafic bodies, and generally have a higher vanadium content (which gives a slightly bluish cast to the green).
      Funny thing about the naming of lesser gems, much “topázio” in Brazil is heat treated amethyst, which you could even call fake citrine. On the other hand I once picked up a sizeable deep green tourmaline out of a basket of schorl – it was dark enough that its transparency was not obvious, sometimes the buyer CAN win.

      • Congratulations! You could always flog it as a “dark Paraiba tourmaline”… 😉 You’re quite correct about the genesis of emerald and the Colombian ones are famous for their perfect colour. Other localities such as Brazil, Afghanistan/Pakistan and the Ural mountains are (very) pale in comparison whereas the gems from Austria (Habachtaal) and Norway (Byrud, Eidsvoll) are both small and scarce. Other localities South Africa, Zimbabwe, Mozambique, Namibia, Nigeria, Somalia, Australia and others rarely yield emeralds of good colour.

    • It’s as you say, Diana, a lady hopes that her hero will consider a diamond will enhance her inner beauty, and some heroes don’t see the need for diamonds to do that….

      • Alyson…… My hero loves me for what I am as you so rightly point out. I have already passed on any gems to be cared for until my Grandchildren are of an age when they will be appreciated…. Diamonds and digging up potatoes or weeding and washing do not go together! But I do have a beautiful collection of amber and my favourite bracelet made from delicately braided silver threads and amethysts may not be, gem wise, valuable but the sentiments associated with it are worth more to me than the Koh-I-Noor Diamond.

  4. Thanks Henrik! This post is a gem itself. One to read and re-read. I think diamond is the only gem made from a single element. The variety in the rest is absolutely amazing, from the range of compositions, trace elements and growth conditions. how long do gemstones take to form? Diamonds can take forever, perhaps as long as a billion years, but not many places on Earth are left undisturbed that long!

    There was a beautiful BBC program last year on colour on Earth, presented by Czerski. If that is ever repeated, I can recommend it.

    • Although gems have been cut from sulphur, it is only as a curiosity and for collectors as crystalline sulpur is so soft that it can be scratched by a fingernail, so yes. Diamond and Jet (cut from bituminous coal) are the only gems cut from a single element even if the latter is not pure.

      This is an incomplete list of minerals used to make gems from according to mineral group:

      Sulphides
      Cinnober, sphalerite, pyrite, marcasite, proustite

      Halogenides
      Fluorite (Derbyshire Blue John)

      Oxides & Hydroxides
      Cuprite, periclase, corundum (sapphire & ruby), spinel, gahnite, chrysoberyl, hematite, ilmenite, rutile, cassiterite

      Phosphates
      Apatite, amblygonite, lazulite, vivianite

      Sulphates
      Baryte, scheelite

      Carbonates
      Calcite, rhodocrosite, dolomite, aragonite, cerrusite, malachite, azurite

      Borates & Nitrates
      (none)

      Silicates = Le beau reste

    • … and to think, I would be happy with a chunk of shocked quartz from around Montgomery Alabama. (Wetumpka impact site).

      • I guess all summit eruptions would do this. Ony if they migrate to become a flank eruption could it by[pass a breach. Doesn’t look like that happened.

  5. …New earthquake with the magnitude of 3,0 took place in Bárðarbunga volcano at 01:24 UTC on 05-January-2016. The earthquakes that have been taking place have been low period earthquakes based on the fact they are not recording properly with the SIL network (tectonic earthquakes almost always have correct magnitude).

    This is a sharp increase in earthquake activity in Bárðarbunga volcano. Currently nothing suggests that eruption is about to start, due to lack of continuous earthquake swarm in Bárðarbunga volcano. This earthquake activity however suggests that magma pressure is increasing fast in a magma chamber that is located in north-east part of Bárðarbunga volcano caldera…. http://www.jonfr.com/volcano/?p=5981

  6. A nice article about one of my favorite things – pretty, colorful, stones that sparkle and shine 🙂

    • I saw that story. Nightfall works so well because it shows how environment affects culture. It was far ahead of its time. But the BBC story is more doubtful. There is a lack of planets in these clusters: it seems plausible that planets can’t easily form in such dense star clusters. lack of Jupiter-like planets is maybe not a major concern: the story that Jupiter protects us against comet impact may not be true. It can have the opposite effect, throwing comets at us. But even without that, these clusters are not the safest place to live.

      • That’s exactly the reason “Nightfall” caught my imagination as a 12-year old schoolboy in the 7th grade. With stars that close even towards the edges, few stars would have gravitational hegemony over their associated Oort cloud = interstellar, high-velocity comets ought to abound. Both their sizes and potential impact speeds should dwarf Shoemaker-Levy 9 into insignificance.

        • Yes, that puts it well. And stars don’t stay in the outer regions. In these clusters, stars are on orbits that regularly take them through the centre of the cluster. The BBC article talks about planets being able to survive near the star, at a distance where liquid water may be present. That is possible. But it does not look at external effects, such as the one you describe. If these places do exist, their sky would be quite a sight.

          Back to Earth. Volcanoes seem awfully quiet this year?

        • I read Nightfall just the once and I still remember that story 37 years later! A great piece of science fiction.

    • Chance of winning the UK lottery: 1 in 14 million. Odds of being hit by lightning: 1 in 300,000 (less if you live in the UK). Odds of dying from a meteor strike: 1 on 200,000. Odds of being hit by a meteor: 1 in 2 billion. (Try to reconcile the last two numbers! Answer: you don’t have to be hit by a meteor in order to be killed by it. Ask any dinosaur.)

      So playing the lottery is quite safe: the only way to get killed by it is through winning it, and the chances of that are miniscule.

  7. Another deep quakes near Trolladyngja

    Wednesday
    06.01.2016 17:21:12 64.884 -17.191 19.4 km 1.1 99.0 3.1 km ESE of Trölladyngja
    Wednesday
    06.01.2016 17:21:06 64.899 -17.177 18.4 km 1.0 99.0 3.4 km E of Trölladyngja
    Wednesday
    06.01.2016 17:12:53 64.877 -17.195 24.8 km 1.1 99.0 3.3 km SE of Trölladyngja
    Wednesday
    06.01.2016 17:12:38 64.882 -17.182 20.5 km 1.2 99.0 3.5 km ESE of Trölladyngja

    Very similar swarm in this area was on 15.11.2015

    http://www.jonfr.com/volcano/?p=5875

    Maybe another dyke forming?

    • Interesting. It is not exactly the same location as the swarm in November. That one was underneath the saddle point between Trölladyngja and Bardarbunga. This one is closer to Trölladyngja itself, and to the side. Not a dyke, I think, but a bit of magma underneath is on the move. It could be old magma reheated by the effect of last year’s eruption. That is guessing though.

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