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.
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.
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”.
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.
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.
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.
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.
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!)
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.
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.