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Specific Gravity and Other Characteristics
According to the U.S. Geological Survey, feldspar has a specific gravity--or ratio of its density to the density of water--of 2.55 to 2.76. It has a hardness level of 6 to 6.5 on the Mohs scale, which ranks hardness from 1 to 10, with 1 representing talc and 10 representing a diamond. Feldspars are found in all types of rocks: hydrothermal, igneous, sedimentary and metamorphic. There are two major groups of feldspars: the plagioclase series, which includes labradorite, sunstone and peristerite; and the alkali series, which includes moonstone, orthoclase and amazonite.
Labradorite
Labradorite gemstones are colorful--most commonly blue, green or yellow--with a beautiful iridescense, and are occasionally transparent. The gems are cut to maximize their iridescent properties. Labradorite comes from Labrador, Canada, but other similar stones from other locations are also considered to be labradorite. In ancient times, labradorite was believed to bring good luck.
Other Plagioclase Feldspars
Other gems in the plagioclase series are striking as well. Sunstone has a glittering sheen, and peristerite has a blue-white iridescence.
Moonstone
In certain lights, moonstones have a milky blue color sometimes thought to look like moonlight. They can be colorless or have yellow, white and gray in them. The world's best moonstones come from Sri Lanka, where they are mined from deep, narrow pits in the ground and are extracted from the buckets of gravel that the miners bring up from deep below the surface that are washed to reveal the precious gems.
Other Alkali Feldspars
Orthoclases are yellow stones with occasional red. Amazonites have a greenish blue or yellowish green hue and were once worn by gamblers for good luck.