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Peridot is a type of silicate mineral known as olivine, and is most often formed underground and brought with the surface with volcanic activity. In Hawaii, peridot has historically represented the tears of Pele (the volcano goddess of fire) and very small peridot grains can be found on some Hawaiian beaches. Peridot can even be found in some pallasite meteorites! While the mineral olivine is quite common, gem quality peridot is rare. Today the primary source is the San Carlos Reservation in Arizona, but other sources include Burma (Myanmar), China, Tanzania, Sri Lanka, Afghanistan, and more recently, Pakistan, where large flawless crystals with excellent color have been found. The finest color is a rich vibrant green with a slight yellowish tint known as “apple green”. Peridot is an affordable and popular gemstone, and because it has a Mohs hardness of 6.5-7 it can be used in all forms of jewelry.
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Did you ever wonder why two similar gemstones often show a difference in brilliance, scintillation and dispersion, even with the same material and same cut? Commonly known characteristics like cut, clarity or refractive index are all important, but it’s a little understood characteristic called luster that plays a critical role. What is luster? Luster represents quality of the polish (reflectivity), which depends on both the material and the polishing agent. Luster is often confused with brilliance, but these are two completely different characteristics. Generally speaking, hard materials such as metallic compounds or diamonds have the highest luster, and soft materials such as amber have low luster. Pearls have, of course, a “pearly” luster. A high-quality polish facilitates refraction and dispersion at a molecular level at the gem’s surface, subtlety but critically improving the appearance of the gems.
There are two major groups of polishing agents – oxides and diamond. Oxides such as cerium, alumina and chromium have their use for specific purposes (quartz is commonly polished with cerium oxide), but most other stones are successfully and quickly polished with various grits of diamond. Large diameter diamond grits such as 320 or 600 are used for rough cutting, fine grits such as 3000 or 8000 are used for pre-polish, and extra-fine grits from 50,000 to 100,000 are used for polishing. 14,000 is often used as a final polish in overseas cutting houses because it’s fast, but it leaves fine scratches and a “greasy” look on the facet surfaces that can inhibit refraction and dispersion. A 100,000 polish takes more time and skill, so it’s often used only for gemcutting competitions. Many cutters will say that customers can’t tell the difference in polish, but we disagree! All Finewater-cut stones except quartz are polished with 100,000 grit diamond, so you can always be assured of the finest possible polish and luster. |
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