gemstones

gemstones Gemstones are minerals esteemed for their qualities of beauty, durability, and rarity. Beauty is generally perceived as a good clear colour and transparency, as shown by fine emeralds or rubies. Beauty may also result from a play of colours or an attractive sheen, as in diamond and precious opal. The dispersion or ‘fire’ of a colourless diamond is due to the splitting of white light into its spectral colours as it is reflected inside the stone (Fig. 1). In precious opal, the minute spheres of silica which make up its internal structure act as a diffraction grating, splitting light into its spectral colours. ‘Star stones’, which include some sapphires and rubies, show asterism, a star-shaped reflection of light from orientated fine needle-shaped inclusions in the stone. Reflection of light from parallel inclusions or a fibrous structure, known as chatoyancy, is shown best by the cat's-eye variety of chrysoberyl.

Although soft or fragile minerals can make exquisite cut stones for the collector, the ability to withstand chemical attack and the abrasion of everyday use is essential for gemstones used in jewellery. When measured on Mohs' scale of hardness (a scale of ten points from the softest, talc (1), to the hardest, diamond (10) ), most gemstones measure at least 7 and so are at least as hard as quartz. Even hard gemstones may be vulnerable to abrasion. The brittleness of zircon is often revealed by chipping on its facet edges, while topaz will readily split along its perfect basal cleavage.

The most highly prized gems are those rare in nature; the larger and more flawless the stone, the greater its rarity and hence the higher the value attributed to it. The quality of diamond in particular and of gemstones in general is assessed according to the ‘four C's’: colour, clarity, cut, and carat weight (5 carats = 1 gram). The world's largest faceted diamond is the yellow ‘Golden Jubilee’, unveiled in 1995 and weighing 545.67 carats.

Gems and gem deposits

Gem minerals are formed in a very wide range of geological environments. Igneous hosts include volcanic pipes and basalt flows, but particularly important are pegmatites. Formed from slow-cooling volatile-rich magma, these coarse-grained rocks are the source of beryl, corundum, tourmaline, topaz, and many other precious stones. Garnet, emerald, jade, and lapis lazuli are among the gemstones created by metamorphic processes. All the more durable gems may be found concentrated in alluvial and beach deposits following erosion of their host rocks. Discovery of these gem gravels has often preceded the search for the source rock and its subsequent exploitation.

The following accounts of the main precious and semiprecious stones give only a few of their most important sources; Robert Webster expands on this subject in his book (revised by Read). New gem deposits continue to be discovered, particularly in the countries of the former USSR, China, and remoter areas of Asia and Africa.

Diamond is composed of pure carbon and is the hardest material known to man. Familiar in its colourless form, it also occurs in a wide range of strong colours including blue, green, pink, and yellow. These rare coloured stones are highly treasured; their coloration comes from traces of elements such as boron and nitrogen or from structural flaws in the crystal lattice. Diamond is obtained from volcanic pipes composed of kimberlite or lamproite, rocks that are found only in cratons, very old stable areas of the Earth's crust. The deposits of South Africa are well known, but most diamond today comes from Australia, Botswana, Zaire, and Russia.

Volcanic rocks in parts of Hungary (now Slovakia) were the main source of precious opal (hydrated silicon dioxide) from Roman times until the discovery of the Australian opal fields in the nineteenth century. Australian opal is formed by the circulation of low-temperature silica-rich water in sedimentary rocks. It occurs as nodules and thin bands and sometimes replaces organic material in marine fossils. Australia yields the bulk of the world's supply of opal, the remainder coming mainly from Mexico and Brazil.

Gem varieties of beryl (beryllium aluminium silicate) include emerald (bright green), aquamarine (blue-green), heliodor (yellow), goshenite (colourless), and morganite (pink). Beryl is derived mainly from granite pegmatites and alluvial deposits, although gem deposits of emerald are also found in schists, gneisses, and hydrothermal veins. By far the finest emeralds come from Colombia.

Ruby and sapphire are gem varieties of corundum (aluminium oxide). The name ‘sapphire’ is given to all colours excluding red and includes green, yellow, and colourless stones as well as the more familiar blue. The finest rubies are of metamorphic origin, occurring in marbles and gem gravels in the Mogok area of Burma. Sapphires are also found in Burma, where they occur in syenites and pegmatites. Sapphire and ruby are obtained from basaltic rocks, pegmatites, and limestones in several countries of Asia, including Thailand, Cambodia, Vietnam, India, Sri Lanka, and, more recently, China. Australia and some countries in Africa and America also produce corundum of gem quality.

Cat's-eye (cymophane) and alexandrite are rare and much sought after varieties of chrysoberyl (beryllium aluminium oxide), a mineral often found in the same gem deposits as ruby and sapphire, together with spinel (magnesium aluminium oxide) and zircon (zirconium silicate).

The finest cat's-eyes are honey-coloured and come from the gem gravels of Sri Lanka. Alexandrites from Sri Lanka, Brazil, Zimbabwe, Madagascar, and other modern sources rarely show the extraordinary colour change of this mineral so well as the old Russian gems. It appears red under tungsten lighting and green by daylight because it contains trace amounts of chromium, the same element that colours rubies red and emeralds green.

Spinel may be blue, purple, pink, or red, while the colour range of zircon includes yellow, green, red, orange, and brown. The high refractive index of zircon gives it a particularly bright lustre.

Gem-quality olivine (magnesium iron silicate), known as peridot, is an oily olive-green colour. Although olivine is an important constituent of many mafic igneous rocks, large crystals are rare. St John's Island (Zebirget) in the Red Sea yielded the peridot brought to Europe by crusaders in the Middle Ages. Today, some of the finest crystals come from Pakistan and Brazil.

Brazil is the main source of gem topaz (an aluminium silicate fluoride in which hydroxyl ions replace some of the fluorine ions). Topaz can be colourless, blue, and rarely pink as well as the more familiar yellow. The colour range of tourmaline (a group of complex borosilicates) is exceptionally wide, and reflects its considerable variation in composition. Single crystals of two or more different colours are not uncommon, making unusual multicoloured faceted stones.

Amethyst, rock crystal, citrine, rose quartz, and smoky quartz are all varieties of quartz (silicon dioxide). Most gem quartz comes from pegmatite deposits worldwide, but especially from Brazil, Madagascar, and Namibia. Large amounts of amethyst are also obtained from geodes in volcanic lavas in Brazil and Uruguay. Crocidolite (blue asbestos) replaced by quartz, known as tiger's-eye, comes mainly from South Africa and Australia.

Garnets are a structurally related group of silicate minerals of various compositions. They include the more familiar almandine (deep pink) and pyrope (red) as well as grossular (pink, green, colourless, and the orange variety known as hessonite), spessartine (orange-red), and the demantoid variety of andradite (bright green). Most garnets form as a result of metamorphic processes.

Metamorphic environments also yield some of the finest gemstones for carving: jade and lapis lazuli. The term ‘jade’ is applied to two different materials, jadeite and nephrite. Jadeite (sodium aluminium silicate) is the rarer, harder, and more highly prized form. Its colour range is diverse and includes the prized emerald-green imperial jade. Most jadeite has come from Burma and Guatemala. Nephrite jade is the compact variety of the closely related minerals tremolite and actinolite (calcium magnesium iron aluminium silicate). It ranges in colour from mid- and pale green to white and it is obtained from Burma, China, and elsewhere. Nephrite jade from New Zealand has long been used by the Maori people to make tools and weapons.

Lapis lazuli is an ornamental rock formed by the regional metamorphism of limestone; it has blue lazurite (sodium calcium aluminosilicate sulphate), white calcite, and pyrite as its main constituents. Most lapis lazuli today comes from Afghanistan, a source since antiquity, although it is also obtained from the Chile, Russia, and elsewhere.

The term ‘gemstone’ also encompasses some materials of organic origin. These include jet (fossil wood), amber (fossil tree resin), coral, pearl, and ivory.

The cutting of gemstones

Translucent and opaque stones, and those showing asterism or chatoyancy, are usually cut in the cabochon style, with a polished domed surface. Other gems are faceted with flat faces, a process that requires great skill and precision to maximize both the beauty and the size of the finished stone.

The process takes place in three stages. First the crystal is sawn using a blade impregnated with diamond powder to remove flawed sections and form the initial shape. The faces are then ground on a lap, a fast-revolving horizontal metal disc charged with a mixture of abrasive and oil or water. The stone is mounted on a dop-stick and held against the lap at precise angles to grind each face in turn, first the table facet, then the crown facets, and lastly the pavilion facets (Fig. 2). Finally each face is polished on the lap using various polishing agents.

Because of their extreme hardness and high value, diamonds are prepared for faceting by slightly different methods. Where possible, flawed sections are removed and larger crystals cut into smaller pieces with minimal wastage by splitting the crystal along natural cleavage planes. First, a notch or kerf is cut using a laser or another diamond. A metal blade is set in the kerf and this is tapped to split the stone. Any cutting needed in other directions is carried out using either a very thin diamond-charged saw blade or a laser, the latter being very much faster. The stone is next mounted on a lathe and, using a second diamond, the points of the crystal are ground away to form a round girdle, a process known as bruting. The individual faces are then ground and polished on a lap using diamond powder as an abrasive. In the past, each stage of diamond cutting was carried out by hand, but the process has become increasingly automated in recent years, benefiting from computer technologies.

The style of cut of any gemstone will depend on the shape and size of the crystal, the intensity of colour, and on other optical properties of the mineral. Styles range from variants of the traditional rose, brilliant, and step cuts more commonly used in jewellery (Fig. 3) to a multitude of modern designs, some of striking appearance.

The brilliant cut was reputedly introduced by Vicenzio Peruzzi in the seventeenth century; it was refined by Marcel Tolkowsky, who published his design in 1914. It has 58 facets, the proportions and angles chosen to optimize the fire and brilliance of a diamond or any stone of high optical dispersion. The zircon cut has an extra set of facets to reduce the loss of light through the pavilion. The step (or trap) cut is particularly well suited to coloured stones, the depth of the gem regulating the intensity of the stone's colour. The truncated corners of the emerald cut give the stone an octagonal outline and help to reduce risk of abrasion. The scissor or cross cut is a form of step cut with triangular faces. Small stones step cut in an elongate rectangular shape are known as baguettes. The benefits of two cuts are obtained for stones of high refractive index and good colour with a mixed cut of brilliant crown and step cut pavilion.

Enhanced, simulant, and synthetic gemstones

The enhancement of natural gemstones to give a better colour or clarity has a long history. In his Natural History, Pliny the Elder (ad 23–79) describe the oiling of ‘smaragdi’ (emeralds and other green stones) to disguise flaws and enhance their colour; today oils, waxes, plastics, and resins are widely used. Flaws in diamonds may be burnt out with lasers, while natural fractures in diamonds and other gems can be filled with a glassy material. As the materials applied are less durable than the mineral itself and may degrade in an unsightly way, such treatments do not enhance the value or desirability of the stone, and their detection is important.

Traditional methods of colour enhancement include painting or applying foil to the backs of stones, and the dyeing or bleaching of porous gems such as agate. Heating under controlled conditions or irradiating a mineral using X-rays, neutrons, gamma rays, or other energy sources will effect colour changes in many gems. Sometimes these processes simulate the effects of natural heating and radiation within the Earth, but in others they produce colours unknown in nature. The blue zircon seen in jewellery stores is obtained only by heating brown crystals. Similarly, citrine, though uncommon in nature, is widely available in the form of heat-treated amethyst. The colour of sapphires and other gems may be deepened by controlled heating. Much smoky quartz is produced by irradiating rock crystal, and irradiation generates the intense blue colour of some topaz gems. Even diamonds may be irradiated to alter or enhance their colour. The regulations relating to disclosure of gemstone enhancements vary from country to country.

The imitation of gemstones by less costly materials also has a long history. Coloured glasses, known as pastes, have been widely used as crude imitations of many gemstones, and synthetic spinel is manufactured in a variety of colours for the same purpose. The quest for a good simulant of diamond has led to the production of a number of materials matching ever more closely its optical and physical properties. These include colourless zircon, strontium titanate, yttrium aluminium garnet (YAG), colourless sapphire, cubic zirconia, and, most recently, synthetic moissanite (colourless silicon carbide).

Doublets and triplets are composite stones consisting of two or three layers of natural or manufactured gem materials cemented together. Some composites such as precious opal protected between layers of quartz or glass are sold as such. Others, typically a coloured glass base and natural gem top, are intended to deceive, and may be difficult to detect when set in jewellery.

The best simulants of natural gemstones are synthetic materials manufactured under laboratory conditions, identical in both chemical composition and crystal structure to their more costly natural counterparts. They can be distinguished by small variations in their physical properties. Synthetic ruby, sapphire, spinel, emerald, opal, and turquoise are commonly encountered, but synthesis of diamond for gem cutting has so far been very limited.

The mechanisms for enhancement of gemstones are explained in the book by Nassau, and accounts of many enhanced, simulant, and synthetic stones are given in O'Donoghue's book.

Identifying gemstones

Only non-destructive testing can be used to identify a gemstone and detect simulant and synthetic stones. Methods include visual examination, study of the absorption spectrum, and measurement of refractive indices, specific gravity, and thermal conductivities. Read's Gemmology (1999) explains these and other tests in detail.

Close visual examination of a stone using a hand lens or microscope will reveal any microscopic inclusions of crystals, liquids, and gases, the shapes and characters of which may not only assist with identification of the stone, but may also show whether it is natural or synthetic, and in some cases may indicate its geographical origins. Zoning or patchiness of colour distribution may also provide useful clues. Examination may also reveal a double image of the pavilion facets seen though the table facet, an indication that the stone is not diamond, garnet, spinel, or any other mineral belonging to the cubic system of crystal symmetry.

Minerals of the cubic system are isotropic, have just one refractive index, and do not polarize light passing through them. Those of the other six crystal systems are doubly refractive. They polarize light passing through them, the polarized rays having different refractive indices. For most gemstones the refractive index can be measured with a gemmologist's refractometer, but the refractive indices of diamond, some zircons, and certain other gem materials are too high to be measured with most of these instruments.

The emerging polarized rays from a doubly refracting stone may be of different colours, a property known as pleochroism. This can be observed with a dichroscope, which splits the emergent rays and shows them side by side. Not all doubly refracting stones show pleochroism but for those that do, the effect can be striking. Cut stones of iolite, the blue gem variety of cordierite (magnesium iron aluminium silicate), show a marked pleochroism of yellow, light blue, and dark violet-blue, distinguishing them from sapphire, which shows a dark blue–pale greenish-blue pleochroism.

The absorption spectrum of gemstones in the visible light region of the electromagnetic spectrum is observed through a spectroscope, and appears as fine dark bands. The positions of these bands provide information about the chemical composition of the stone and may also indicate whether the stone is synthetic or artificially modified. Any fluorescence under ultraviolet light may also provide clues to the identity and status of a gemstone.

The specific gravity (the ratio of its weight to the weight of an equal volume of pure water at atmospheric pressure and a temperature of 4°C) of an unmounted gemstone can be gauged using an accurate balance and applying Archimedes' principle. It may also be estimated by immersing the stone in a series of liquids of known specific gravities, to see whether it sinks, remains suspended, or floats.

None of the tests mentioned above can identify a diamond in jewellery with absolute certainty. One of the most effective tests is measurement of thermal conductivity using a small electronic probe. Conductivity is particularly high for diamond but low for nearly all its simulants.

Monica T. Price

Bibliography

Harlow, G. E. (ed.) (1998) The nature of diamonds. Cambridge University Press.
Nassau, K. (1994) Gemstone enhancement (2nd edn). Butterworth-Heinemann, London.
O'Donoghue, M. (1997) Synthetic, imitation and treated gemstones. Butterworth-Heinemann, London.
Read, P. G. (1999) Gemmology (2nd edn). Butterworth-Heinemann, London.
Webster, R. (1994) Gems: their sources, descriptions and identification (5th edn revised by P. G. Read ). Butterworth-Heinemann, London.