Gemstones: Chemistry, Quality, and Settings

Most gemstones are crystals Some are composed of organic materials such as pearls. shell and corals. The crystalline structure and the presence of minerals condition their cut, color, and hardness. The type of cut affects the style of setting.

The Mohs Scale:

The relative hardness of gemstones is described by the number given to them on the Mohs Scale.
  1. Talc
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Orthoclase
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond
Each material will scratch those with a smaller number and be scratched by those with a higher number. Diamonds are 80 times harder than corundum so the steps of the scale are not regular. In household terms window glass, for example, is 5.5 on the Mohs scale, a fingernail is 2.5, a file is 6.5. With the exception of organics like pearls (3 on the Mohs scale) or ivory (2) most gemstones used in jewelry are 5 and higher.

Color , Clarity, and Carat:

In general the fewer inclusions (internal defects or mineral deposits) a gems1.htmltone has the finer quality it is considered to be, but color also determines the price. Emeralds, for example, that are relatively clear (almost no emeralds are completely free of inclusions) and a specific "emerald green", are more expensive than off color and included stones. In some cases, however, high quality stones, such as sapphires and amethysts, do not look as good cut as lower quality stones. The higher priced stones are much darker.

Color, Clarity, Cut and Carat are the Four Cs of diamond buying. There are 5 carats to a gram. Each carat is divided into 100 points. Somewhat confusingly, the fineness (amount of pure metal) of gold is also defined by karat, but here spelled with a k not a c. Unless we're discussing diamonds I usually refer to the size of stones by millimeter rather than carat.

Types of Settings:

We generally set cabachon gemstones in bezel settings, a style of setting where a thin band of sterling silver or 14 karat gold completely surrounds the stone. We generally set faceted stones in prong settings, of which there's a large variety. In prong settings metal prongs are bent over the girdle of the stone and hold it in place. Faceted stones can also be set in bezel settings.
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