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.