Peridot: The Gem Form of Olivine

Olivine is normally an opaque rock, but as peridot can sometimes be found as translucent stones or crystals. Normally it is associated with either basalt or peridotite. Small crystals form near these rocks while it cools slowly deep in the earth. The slower this molten rock cools the larger the peridot crystals can grow.

The gem gravels of Burma sometimes produce large, nearly transparent stones of peridot. This is the main source of this stone, it is also found on Zebirget Island in the Red Sea. Another more recent source is Arizona, USA.

Peridot or Chrysolite is August's birthstone and it is also associated with the sun sign Libra, the apostle St. Bartholomew, and the hour of 7AM.

Description:

Peridot is the name applied by jewelers to the mineral known as olivine. The term chrysolite has also been applied to this material, especially to the pale-colored varieties. Gem-quality peridot is transparent and occurs in yellowish green, green, greenish yellow, brownish green, and brown. It can be confused with demantoid garnet (often called ‘olivine’ by jewelers), emerald, tourmaline, chrysoberyl, zircon, sapphire, synthetic sapphire, synthetic spinel, doublets, and artificial and natural glass (moldavite).

Peridot is a magnesium-iron silicate, which in fine quality, contains much more magnesium than iron. Since the proportions vary in different deposits, some property variation is expected. In the usual green or rare brown, gem peridot is usually near 1.654 and 1.690 in refractive indices, with a birefringence near .036. The specific gravity is usually near 3.32 to 3.35, but slightly higher readings are occasionally encountered.

Sources:

Peridot forms in metamorphic contact zones where high heat penetrates a layer of cooler sedimentary rock. Intrusive granitic magma increases the temperature of the solid rock to more than 1800 degrees Fahrenheit. Excited by the heat energy, the atoms in the sedimentary mineral break free of the crystal structure and reform into new, more stable structures.

Technical Information
Refractive Index:	1.654 +/- .020
Birefringence:	0.036
Specific Gravity:	3.34 +.14 -.03
Pleochroism:	Weak
Formula:	(MgFe)2SiO4
Fracture:	Conchoidal to uneven
Mohs Hardness:	6.5 - 7

Bibliography

Handbook of Gem Identification by Richard Liddicoat

The Curious Lore of Precious stones by George Frederick Kunz