HISTORY OF GEMS

The history of gemstones is rich with stories of adventures, of legends, of tales of good fortune or curses resulting in financial ruin, ill health, or even death. Throughout antiquity, gemstones have been worn as a talisman, valued for their healing properties and endowed with spiritual values. The magical and mystical properties of the gemstones have much to do with their rarity, beauty, feel, and colour.

Gems have long been seen as exotic, rare and valuable. Many were carried long distances along perilous trade routes from distant and unknown lands, which added to their wonder and their value. As different gemstones have become available, fashions have changed and preferences have varied worldwide. Nowadays, a diamond is thought of as the ultimate gem, worn in engagement rings and given as a token of love, but this has not always been the case.

At various times in the past, turquoise, amethyst, lapis lazuli, jasper, and carnelian have all been regarded as the ultimate gem. jade was a favourite in China and Mexico. The ancient Egyptians and the civilizations of Central and South America valued emeralds. Emer­alds, sapphires, amethyst, jasper and carnelian were the Romans' pre­ferred choice, while diamonds were used to engrave cameos rather than be worn as jewellery.

Many of the stories associated with gemstones have been handed down by word of mouth or from the diaries and letters of travellers or collectors or as entries in the inventories of private collections, museums or royalty. In the 13th century, Marco Polo (c. 1254-1324), a trader from Venice, travelled to Asia and wrote in his journal, The Book of Marvels, that he carried sapphires as calling cards when he visited the court of Kublai Khan, the Mongol Emperor. The sapphires were from southwest Sri Lanka (formerly Ceylon), from the area around Ratnapura ('City of Gems' in Sinhalese). In the 17th century, the French merchant Jean-BaptisteTavernier (1605-89) made his fortune by trading in gemstones. Tavernier made six trips to India and Persia (now Iran} between 1631 and 1668, described many large diamonds and acquired a number of gems, some of which were sold to King Louis XIV of France.

Most of the famous, named gemstones are dia­monds. Renaming and recutting as ownership changes may complicate their history and the secrecy surrounding some gems and their whereabouts makes confirmation of size, shape and weight difficult if not impossible. Museum specimens can be researched and some famous diamonds can be recognized from paintings or photographs, but those that are bought at auction by an 'unknown private buyer', or those that are lost or are the victims of theft, simply 'disappear', sometimes for many years. The azure 'Nassak' (now 43 carats but originally 90 carats), also known as The Eye of the Idol', was placed in the forehead of a statue of Shiva at a tem­ple in Nassik, Maharashtra, India, but disappeared when British troops looted the temple in 1818. In 1927 it resurfaced and was recut in New York.

The oldest diamonds with the longest histories largely originate from the alluvial deposits of the Golconda region-of south-central India. They include the Koh-i-Noor, Orlov, Regent (Pitt), and Hope diamonds. Some of the largest and most famous diamonds are from the Premier Mine in South Africa, including the Cullinan and the Taylor-Burton (cut 69.42 carats). The largest diamonds in the world, the Golden Jubilee or Unnamed Brown (545.67 carats) and the De Beers Millennium Star (203 carats, which took ten people two years to cut) are both African. In 1988 the Centenary diamond (599 carats uncut, 273.85 carats cut) was cut by the company De Beers to celebrate the 100th anniversary of its De Beers Consolidated mining operations.

Famous coloured diamonds include the blue Hope diamond, the Dresden Green, and the golden-yellow Tiffany diamond (cut 128.54 carats). Other blue diamonds include the Townshend Blue (in the Victo­ria and Albert Museum, London), and the aforementioned pale blue Nassak. The Dresden Green is the world's largest pear-shaped green diamond (41 carats) and, apart from occasional loans, it has been kept in the vaults at Dresden Palace since its purchase by Frederick Augustus II of Saxony for US$150,000 at a Leipzig fair in 1743.

Today, pink diamonds from the Argyle mines in Western Aus­tralia are particularly prized. In 1986, a huge diamond referred to as the 'Unnamed Brown' was used by De Beers to test their new laser cutting technology. The diamond weighed 755.50 carats when rough and 545.7 carats once cut. It was renamed the 'Golden Jubilee' after its presentation to King Rama IX of Thailand in celebration of 50 years on the throne. Another brown diamond is the 'Incomparable' (407 carats) found in the Democratic Republic of Congo (formerly Zaire) in 1980. Black diamonds have been cut weighing more than 115 .carats. The Black Orlov or 'Eye of Brahma' cushion-cut black diamond ,67.50 carats, is said to have been stolen from a shrine in Pondicherry, southern India, and to have weighed 195 carats in the rough.

Other famous gemstones include the red spinels known as the Black Prince's ruby (in the British crown jewels) and the Kuwait ruby (formerly theTimur ruby), Saint Edward's sapphire and the Stuart sapphire (both In the British crown jewels), the Devonshire emerald, the Edwardes ruby, and the Rosser Reeves and Appalachian Star rubies.

ENHANCEMENT

The colour and appearance of natural and synthetic gemstones can be enhanced by methods such as oiling, dying, staining, foiling, heat-treating, iridating, drilling and filling. Heavily included gemstones, such as emerald, are oiled to fill cracks and give the stone a clearer appearance. Oiling has been carried out for more than 2000 years, and is an acceptable part of the business. Filling the cracks with oil, wax and stain may be temporary, but more long lasting resins have recently be introduced.

Oils and stains may leak and can be identified by wiping with a soft cotton cloth. Turquoise, Lapis Lazuli, Jade and Agates may be given a coating of wax. Agates are often stained to bright colours that have no natural counterpart and are therefore easily recognized as fakes, though some find them attractive. Placing a piece of foil behind a mounted gemstone to increase its fire and sparkle was a method used by Victorians, particularly for costume jewellery. Probably the oldest method of enhancing the appearance of a gemstone is by heating. In India, Carnelian has been heat-treated for more than 4000 years. Heating may be carried out in oxidizing conditions or in reducing conditions. Heat treating and irradiating gemstones can be used to change or enhance colour. For example, colourless and pale brown Topaz can be heat-treated to give blue Topaz. In addition lasers can be used to drill even the hardest diamonds to remove inclusions and the drill hole may then be filled.

INCLUSIONS IN SYNTHETIC GEMSTONES

A perfectly clear stone, without flaws or inclusions (internal features of gemstones, such as solids, liquids or gases, that have become enclosed within the gemstone during or after formation) is usually a warning sign to a gemologist that the stone may be synthetic. However some synthetic gems may be identified by the type of inclusions, formed as a result of the method of manufacture. Flame-fusion gem-stones may include specks of the powdered ingredients that have failed to melt sufficiently, and have curved growth-lines, rather than the straight lines that develop in natural specimens.

Synthetic-flux emeralds may have inclusions of the minerals phenakite or platinum, twisted veils and feathers (characteristic patterns of inclusions), and two-phase inclu­sions (a liquid and a solid). Synthetic hydrothermal emeralds usually have only a few inclusions, possibly of phenakite and occasionally very fine two-phase inclusions.

natural inclusions

Natural inclusions may be specific to one gem or even to a particular country or mine. Emeralds, for example, tend to be so heavily included that the view down a microscope (X40 magnifica­tion) is sometimes referred to as jardin (French, 'garden'). Natural emeralds may have inclusions of the minerals mica, tremolite, actinolite, pyrite, or calcite. Colombian emeralds may contain characteristic three-phase inclusions (solid, liquid, and gas), with a cubic salt crystal and a gas bub­ble within a fluid-filled cavity. Other gemstones with characteristic inclusions include peridot, which has inclusions that resemble 'lily pads' (liq­uid droplets around a crystal of chromite) and the 'treacly' appearance of hessonite garnet.

SYNTHETICS

Gemstones can also be made in the laboratory. Where they have the same chemical composition as their natural counterpart, they are referred to as synthetic. Synthetic gemstones therefore have virtually the same physical and optical properties of the natural gemstone.

The ruby was the first gemstone to be made artificially. In 1837 a French Chemist Marc.A.Gaudin tried to produce some synthetic rubies but eventually gave up, admitting defeat in the published notes of his final ruby experiments as they were not of any value as gems because they became opaque as they cooled.

In 1877, the French Chemist Edmond Fremy (1814-94) and a student assistant developed a method what is now known as Flame Fusion method. They heated 44.1-66.15 lbs (20-30 kg) of a solution of Aluminium Oxide dissolved in Lead Oxide in a porcelain vat for 20 days. As the solvent evaporated and che,mical reactions took place among the solution, the vessel and furnace gases, a large number of very small Ruby crystals formed on the basin's wall but the crystals could not be used in jewellery as they were very small.

Later in 1891, a French Scientist Auguste Verneuil (1856-1913) developed a somewhat different process that eventually became successful. He did not publish a description of his technique until 1902. His assistant exhibited the synthetic rubies in 1900 at the paris World's Fair, where they were quite popular. By the time Verneuil died at the age of 57 in 1913, the process he had invented was being used to manufacture 10 million carats of rubies annually.

COMPOSITES - GARNET

composites - garnet-topped doublets and soude emeralds

Imitations may also be composites - made of more than one part. For example, a piece of green glass topped with a slice of red garnet can be used to imitate an emerald or green garnet. The garnet-topped doublet (GTD) is made of two parts and the junction between them can usually be recognized by the difference in lustre. There may also be char­acteristic bubbles in the glass, which are not visible in the garnet.

When viewed through the table facet, the gemstone appears green, but when viewed from the side, or immersed in water, the red slice of garnet will be more obvious. GTD can be made to imitate gemstones of all colours by altering the colour of the glass beneath the gemstone top. Another composite is the soude emerald, made of two layers of colourless quartz, sandwiching a thin green layer of gelatine or glass.

opal doublets and opal triplets

opal often occurs in thin seams with the precious opal forming just a thin sliver or slice. An opal doublet (made of two parts) is made by cementing a piece of precious opal (showing a play of colour) to a 'backing' layer of 'potch' opal (non-precious opal). Opal doublets may also have a quartz, chalcedony, glass, or plastic base. Opal triplets have a protective cover­ing of quartz above the opal in addition to a backing layer.

opal simulants

The play of colour seen in precious opals is due to interference of light by its internal spherical structure. In 1974 the French scientist Pierre Gilson unveiled the first laboratory-produced opal. Gilson imitation opals can be distinguished from natural opals by their patchy appearance and the mosaic-like junctions between coloured 'grains'. The US scientist John Slocum created a glass opal, known as 'Slocum stone'. The patches of colour in Slocum stones have a slightly crumpled look when magnified.

DIAMOND SIMULANTS

One gemstone can be used to imitate another more expensive vari­ety, for example citrine to imitate yellow sapphire, or colourless glass or quartz or American Diamond to imitate diamond. Colourless glass does not make a convincing diamond imitation as it lacks the hardness, fire and sparkle but no one dares to see the hardness of the diamond if it a glass.

Other diamond imitations include CZ (cubic zirconia) and, more recently, moissanite. Moissanite is almost as hard as diamond, mea­suring more than nine on the Mohs' scale. The main difference is that diamond is singly refractive, while moissanite is doubly refractive. In larger moissonite gemstones, this can be seen as a doubling of the pavilion facets when viewed through the stone, but in small stones mounted in jewellery, moissanite can be difficult to recognize and they are seldom identified.

There have been other diamond simulants, including for example yittrium aluminium garnet (YAG) and strontium titanate, but all have either lacked fire, or had too much been too soft or too brittle. Diamond simulants can be distinguished from diamond by the fact that they conduct heat far less readily than diamond. Touching the stone with a thermal conductivity meter will alert the gemologist to the imitation.