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Moonstone is the most well-known gemstone variety of orthoclase feldspar, a potassium aluminum silicate. It is a transparent to opaque oligoclase, a variety of plagioclase albite and sheet mica. Moonstone is known to exhibit a distinct sheen under certain lighting conditions, and it is the sheen which renders moonstone one of the most remarkable gemstones available today. In fact, its name is owed to the almost magical, bluish white shimmer it exhibits, which closely resembles that of the moon. Gemologists refer to the shimmeringoptical phenomena as 'adularescence'.Moonstone can be identified by the presence of adularescence. Other gems with a similar appearance do not have the phenomenal presence of adularescence which makes identification of moonstone fairly easy. Moonstone is a potassium aluminum silicate and can be easily identified by composition. Many similar materials, such as labradorite, are actually plagioclase feldspar, whereas moonstone is by composition a potassium feldspar. Testing for hardness is often one of the easiest methods for distinguishing moonstone from other materials. Other similar gems, such as opal, chalcedony or ammolite, are significantly harder or softer than moonstone. Top quality moonstone can show an incredible "three-dimensional" depth of color, which no other gemstone can replicate, making moonstone almost unmistakable.Moonstone deposits are often found as constituents in feldspar-rich granitic and syenitic pegmatites all over the world. The most important moonstone deposits are from Sri Lanka and India. Other notable sources include Australia, Brazil, Germany, India, Myanmar (Burma), Madagascar, Mexico, Norway, Switzerland, Tanzania, and the United States. Sri Lankan moonstone is most famous for its attractive blue colored material, but blue moonstone is becoming increasingly rare. India is known for producing fine 'rainbow moonstone'. Switzerland's Adula Mountains possess the most historically interesting moonstone mines.
