Anyone who’s admired a sparkling diamond ring might wonder how the gemstone came to be so beautiful. Like most gemstones, the original diamond began as a stony lump that was brought up from the earth. Only after it was cut and polished did it get the beauty and brilliance that allowed it to be set on a ring to attract diamond buyers. Even gemstones that are left unfaceted, called cabochons, need to be cut and polished.
What determines how a gem will be cut is its hardness and transparency. Its index of refraction also determines how it will be cut. The index of refraction measures the speed of light passing through the mineral. Gems that have a low index of refraction have a low luster.
When a gem is cut, a lapidist simply grinds away the material until it’s in the shape that’s desired. The gem is then polished. Because it is the hardest substance known, a diamond is polished differently from other gemstones. A diamond can only be cut and polished with another diamond or diamond dust. Other gemstones can be cut and polished with materials softer than diamond.
The index of refraction determines whether or not the stone should be faceted, or cut with many sides. A transparent gem with a high index of refraction is often cut with facets. A diamond is often cut with a “brilliant” cut that has exactly 58 facets, no matter how large or small the diamond is. The light that strikes the top facet is reflected from some of the other facets and enters the gem through still other facets. In a perfectly cut gemstone, the light that enters the top of the gem is reflected up from the back facets.
Diamonds are cut by a machine that has an iron plate full of diamond dust. The diamond is held in place by a dop while it’s cut and polished. A rough diamond can lose as much as 60 percent of its weight while it’s being polished. The entire operation is overseen by a specialist, and it’s up to him or her to make sure that the stone is cut perfectly. Many otherwise valuable diamonds have been ruined by bad cuts.
Gemstones like rubies and emeralds depend mostly on their beautiful colors for their value and they’re often full of flaws and inclusions. These gemstones are often cut into cabochons, which are also quite beautiful. Some cabochons have asterisms, which are star shaped lights within the gemstone. This starry light makes the gems quite valuable.