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CUTTING AND POLISHING

CUTTING AND POLISHING

A skilled lapidary can turn a rough pebble into a sparkling and valuable gemstone. The knowledge needed for this transformation has been built up over many centuries, and today a style of cutting can be selected to display the special qualities of each and every gem.

When deciding how best to cut a gemstone, the lapidary must consider the shape of the rough material and the position of flaws, fractures and inclusions. He or she must also be aware of the mineral’s optical properties and physical properties such as cleavage. It is difficult to produce a good polish parallel to cleavage directions. Pleachroic gems (those that show a different colour or shade of colour from different directions ) should be oriented to show their best colour. However, the cut is often a compromise between displaying the full beauty of a mineral and producing the biggest gemstone possible, since size also affects the value.

Cabochons are the oldest, simplest cuts. Cabochons are round or oval gems with plain, curved surfaces. Still in use today, cabochons display the best the colours and patterns in opaque and translucent stones, and optical effects such as sheen, iridescence, cat’s – eyes and stars.

The faceted styles now used for almost all transparent gems developed much later, becoming important in medieval Europe and India. In these styles the surface of the gem is worked into a pattern of highly polished, flat planes ( facets), which act as mirrors. Some light is reflected from the surface of the crown (top) facets, displaying the luster. Light entering the gem is reflected back through the top of the stone from the pavilion (bottom) facets, displaying the colour and fire. The facets must be precisely angled to bring out the maximum beauty, the angles varying according to the optical properties of each gem mineral. In badly cut stones light leaks out through the pavilion so that colour and are fire are lost.

The brilliant and step cuts are the most familiar styles in modern jewellery. Brilliant cuts were developed to show off the superb luster and fire of diamond, and are also used for many other gems. Step cuts are most effective in stones such as emeralds, where colour si the supreme quality.

THE BRILLIANT CUT

The brilliant cut is probably the most important type of cut of used for gemstones. The number of facets (flat polished faces) and the angles between them are worked out mathematically so that the gemstone looks bright and sparkles.Most diamonds are cut as round brilliants. Each has 58 facets (or sometimes more in a large stone), of which 33 are on the top part of the stone (the crown) and 25 are on the lower part of the stone (the pavilion). The point at the back of the stone may be removed to prevent accidental damage and to leave a small flat circular facet called the culet. Without the culet, the gemstone has only 57 facets.

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