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.

GLASS

Glass is a versatile material as it can be made almost any colour and can be moulded or faceted to give the required appear­ance. However, there are noticeable differences between glass and gems. Glass is generally not as hard as the gemstone that it imitates, and will be able to scratch more easily. Further, its facet edges may appear scratched and worn. There may also be bub­bles or distinctive swirls within the glass, which can be easily seen with the use of a hand lens. A gemologist will also be able to distinguish glass by its single refraction and the value of its refractive index (1.5-1.7), as there are no singly refractive gems with the same refractive index.

IMITATION AND SYNTHETIC STONES

There will always be a market for imitations or cheaper gems because of the value of gemstones and their rarity,. Generally imitation gemstones are made to deceive. They are made from any material, natural or artificial, that looks similar enough to be taken for the real and more expensive gem. Imitations have been made since thousands of years, for example blue faience (glazed day) was used by the Egyptians to imitate turquoise. Romans and Indians used coloured glass to imitate gemstones includ­ing emeraeds and rubies. The Victorians used a number of materials including glass (paste), plastics and resins to imitate both the natural mineral gemstones and the organic gems such as amber and shell.

STAGES IN FACETING A BRILLIANT DIAMOND

The four main diamond-cutting centres in the world are Antwerp, Mumbai (India), New York City, and Tel Aviv. Although much of the process can now be carried out with the aid of computers, there is still a need for the 'eye' of the expert to check the progress and assess the accuracy, make, and overall look of the gemstone.

Stage 1 The designer will decide how best to use the shape of the rough stone, taking into account any flaws or inclusions, the weight, and the overall quality of the stone. The designer may be looking to produce the largest stone possible, or several smaller stones with a larger combined weight. There will usually be a compromise between the number and size of the stones that could be produced and the final yield. The value and quality of the polished diamond will be judged on its colour, cut, clarity, and carat (weight) - the 4C's.

Stage 2 Once the decision has been made how best to use the rough stone, it is marked for the first cut (saw) or cleavage (a break along a plane of weakness related to the atomic structure of the dia­mond). Nowadays, diamond sawing can be carried out by in a facto­ry by computer controlled lasers.

Stage 3 The first step in faceting an octahedral diamond crystal may be to saw off the top of the crystal using a rotating disc coated with a coarse diamond abrasive. A diamond may also be cleaved to divide it in two. In order to cleave a diamond, the diamond is first cemented to a stick (dop), a small groove (kerf) is made along the mark into which is placed a steel blade. The blade is then given a sharp hit with an iron bar to cleave the diamond in two.

Stage 4 The diamond is given a rounded outline by holding it against a revolving cast-iron wheel, known as a 'scaife', coated with a diamond powder paste. This process, called bruting, gives the diamond the appearance of frosted glass. The girdle, the widest part of the stone, which will separate the top (crown) facets from the lower (pavilion) facets, can be seen at the end of this stage.

Stage 5 The facets are then made using the scaife. The flat table facet is ground first and then the bezel facets. The facets are ground and polished in a particular order; first the pavilion main facets, the culet, star facets, upper girdle facets and lower girdle facets. Stage 6 A 'brillianteer' then adds the remaining 24 crown and 16 pavilion facets, bringing the total to 58 facets including the culet. A finer-grained diamond abrasive powder is used for each progres­sive stage of polishing. The diamond is then ready to be mounted in a ring or other piece of jewellery.

CUSHION CUT AND MIXED CUTS

In order to get the best 'make' of a gem and maintain a good weight, the cushion cut is often used. The cushion cut in a gem is a variation of the brilliant cut, with rounded cushion-like appearance. Mixed cuts gems optimize both the colour and the fire of a gemstone by using dif­ferent cuts on the crown and pavilion facets. The most common mixed-cut is a brilliant-cut crown cuprite gem with a step-cut pavilion.

STEP CUTS

Step-cut gems are generally cut to show their colour rather than fire or brilliance. In gemstones that show pleochroism, the gem is oriented and cut to show the best colour, when viewed through the table facet. sapphires and other gems that may show a colour banding are oriented so that the banding is parallel to the steps of the cut and cannot be seen clearly through the table. If there is an area of a stronger colour, it is best placed toward the base of the gemstone, deep within the pavilion, so that the colour appears to flood the whole stone when viewed through the table facet.

The step cut has a square or rectangular table facet and girdle, surrounded by a series of parallel four-sided facets. The number of facets is deter­mined by the size of the gem. In brittle gems, such as emerald, the corners may be easily damaged, so they are usually removed during cutting. The resulting cut is called the emerald cut or modified step-cut. As more of the corner is removed, the gem nears the outline of a regular octagon and may be called an octagonal cut.

FANCY CUTS

Fancy cuts include variations on the round brilliant cut such as the oval, pear or drop-shaped (pendeloque) and boat-shaped (navette or marquise) brilliants in addition to triangular, hexagonal, pentagonal, kite-shaped, etc. With advances in computer technology and lasers, new shapes and styles are entering the market, including computer generated designs and gems engraved by laser.

BRILLIANT CUT

The proportions of the modern brilliant cut were developed by a number of cutters, among whom the most influential was Henry Morse, who in 1860 opened the first diamond cutting factory in the United States at Boston, Massachusetts. Other designs have since been introduced, including the 'round brilliant' of Marcel Tolkowsky, whose book Diamond Design (1919) systematically analysed the optics of a diamond to show the best fire and brilliance. The brilliant cut is also used for many other gemstones. Variations continue to be developed, usually in accordance with the designs for standard proportions of Eppler, Scandinavian Diamond Nomenclature (SCAN DN Cut), and the International Diamond Council (IDC).

Source: Diamond Design (1919); History of Diamonds.

HISTORY OF GEMSTONE CUTS FOR DIAMONDS

In the gemstone trade, diamonds are usually treated separately from other coloured gemstones. They are the hardest gemstone and are generally cut and fashioned by diamond cutters, who will specialize in their manufacture, or even just one stage in the diamond cutting and polishing process.

The earliest diamond cuts probably entailed no more than removing the top of an octahedron crystal, known as the point cut. Another early cut is the table cut, where the point of the octahedron is flattened to a square facet called the table. This cut was popular during the Middle Ages. As cutting techniques developed, further cuts were made and the polishing of facets was introduced. The rose cut, with a flat back and domed and faceted front made of triangular facets, was introduced by the 14th century.

The most common cut for a diamond is the brilliant cut. The first type of brilliant was the single cut (or half cut) with the main table facet surrounded by eight smaller facets on the crown (the upper part of the stone) and eight on the pavilion (the lower part of the stone). During the 17th century, this evolved to the double cut (or Mazarin cut), with 16 crown and pavilion facets and a culet (the small face formed by polishing the point of the pavilion to a flat facet). The number of facets continued to increase. By the middle of the 17th century, the triple cut (or Peruzzi cut), and variations including the Brazilian cut and Lisbon cut, had 58 facets (including the culet) - the same number as in the modern-day brilliant, which was introduced in the 1820s.