Photo of Peridot in Processed & Rough Form
Peridot is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of magnesium iron silicate, their full chemical compound being (Mg,Fe)2SiO4.
Peridot is the greener type of gem-quality specimen of Forsterite-Olivine, which is an important, rock-forming mineral, a silicate of magnesium and iron. Often their names are used interchangeably. The bottle-green/olive green type of olivine is also called olivine. The yellower type of olivine is called Chrysolite.
It forms as thick, tabular crystals, frequently with wedge-shaped terminations. The color ranges from pale yellow-green to pale olive green to dark green.
It has vitreous luster, and is usually transparent. It tends to burst under great stress, therefore is sometimes metal-foiled. Dark stones can be lightened by burning. Rarities are peridot cat's eye and star peridot.
The particular color and luster of peridotare highly characteristic, although some tourmalines, zircons, and chrysoberyls may look much the same. A quick way of distinguishing them is by testing the density.
Peridot in olivine is very widely-distributed in iron- and magnesium-rich igneous rocks. The most important deposits are in the Red Sea on the volcanic island of St. John (Egypt), where they have been mined for 3,500 years. Beautiful crystals can be found on the walls of cavities of weathering peridot rock. Good material can also be obtained from serpentine. There are also quarries in upper Burma, where bright-green well-formed crystals are found. Hawaiian peridot is found on beaches. Stones from Ameklovdalen, Sondmore, Norway, are iron-poor and a bright yellowish green. Gem-quality peridot is found in China in the Zhangjikou-Xuanhua area of Hebei province.
Peridot is often incorrectly spelled as "peridote."
The specific gravity [?] for Peridot is 3.34, its refractive index [?] is 1.64-1.69, and its double refraction [?] is 0.036.
History
Peridot's name derives from the Greek, perhaps referring to the numerous crystal planes of the crystal. It has been known for a very long time. The ancients, including Pliny the Elder, spoke of a gem called "topazos" (corresponding to our word "topaz") which, from the description, appears to be olivine.
It was brought to Europe by the crusaders in the Middle Ages and was used for ecclesiastical purposes. It was used to decorate church robes and plates, and was very popular during the baroque period.
The largest cut peridot weighs 310 carats and was found on the island of St. John (Red Sea, Egypt). In Russia there are some cut peridots which came out of a meteorite which fell in 1749 in east Siberia.
Peridot can be confused with beryl, chrysoberyl, demantoid, diopside, moldavite, prasiolite, prehnite, sinhalite, emerald, synthetic spinel, and tourmaline.
Industrial Usages
Peridot usually has few inclusions, and is best cut in the trap-cut (step) style, althought it can be given all types of mixed cuts, oval, round, and pear-shaped, plus rectangular and square. Gems of several carats are often seen, but very large stones are hardly ever found. Small stones are also cut and arranged in intricate patterns in jewelry, or set in gold. In table and emerald cuts, it can be just as brilliant.
Much appreciated in the past, peridot is the victim of changing fashions and is far less highly prized today. Even exceptionally fine, large stones do not fetch very high prices, and smaller ones are very low-priced. It is not greatly desired by the trade because of its lower hardness.
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