The first mineral you ever examined conscientiously was probably a gemstone, and gems are the objects that most people associate directly with minerals. While mineralogical properties of most gemstones have been discussed earlier, we will summarize here the unique features that make them valuable, including how to prepare a gemstone out of a rough crystal and a review of testing instruments used by gemologists. There will be short sections about the most important gems and their occurrence and in conclusion methods of crystal synthesis will be described.
General comments about gems
Gemstones have been defined as minerals that are highly valued for their beauty, durability, and rarity; they may be worn for adornment or used to decorate art objects. Since they are rare, they have a high value. The value of gem diamond exceeds the price of gold (by weight unit corresponding to a Troy ounce) by a factor of about 200 (Table 34.1).
Note that in many cases the names used for gems are different from those of the regular minerals that they represent. For example, ruby and sapphire are varieties of corundum, emerald and aquamarine are varieties of beryl, and alexandrite is a variety of chrysoberyl. In addition, the weight units used in gemology are different from those used in ordinary science. The most common weight unit is the carat, which corresponds to 0.2 g. Table 34.2 lists some of the important minerals that are used as gemstones.
The visual appeal of different gemstones varies greatly. The majority are highly transparent to light; others, such as moonstone, opal, or tiger-eye, are translucent; and a few, such as lapis lazuli and jade, are opaque. Most gemstones are colored, and the color gives gems such as ruby, sapphire, emerald, and opal their visual appeal. Diamond is generally a colorless gemstone, and the attractiveness of a particular diamond depends on the interaction of white light with the crystal by internal reflection and dispersion. Many gemstones are single crystals. Exceptions include opal, which is amorphous, and jade, which is a polycrystalline aggregate.
Many gems are literally permanent and do not degrade in the atmosphere, in water, or by abrasion (although not all gems have the high hardness of diamond or corundum).