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Peridot is the traditional birthstone for the month of August.
While the name olivine may be unfamiliar, most people have heard of its gemstone variety: peridot, which has been mined by Egyptians for over 3,500 years on the former "Topazios" then "John's Island" in the Red Sea, now called Zebirget. The stones were only mined at nighttime because it was believed they were not easily seen in daylight. Mining at night was also likely a result of the island being infested with snakes.
"Olivine" is applied to any mineral belonging to the forsterite-fayalite solid-solution series, in which iron and magnesium substitute freely in the structure. Fayalite (Fe2+2SiO4) is the iron end member, and forsterite (Mg2SiO4) is the magnesium end member. The most common intermediate olivines have compositions near the forsterite end of the series, and most peridot is about 90% forsterite.
Crystals are tabular, often with wedge-shaped terminations--although well-formed crystals are rare. Olivine can also be massive or granular. Forsterite-fayalite olivines are usually yellowish-green but can be yellow, brown, or gray. In addition to the usual green color of common olivine--a result of the small amounts of iron that are almost invariably present--its color can vary from white for pure forsterite to black for fatalite, but both are very unusual.
Olivine is inferred to be a major component of Earth's upper mantle, and as such is probably one of the most abundant mineral constituents of the planet. Olivine has also been found in some lunar rocks and in stony and stony-iron meteorites. It is generally the first mineral to crystalize from rocks with relatively low silica content; thus, its composition reflects to some extent that of its parent magma. Thick accumulations can occur as a result of olivine crystals settling through a body of magma that is still partially magnetic. Olivine occurs most commonly in mafic and ultramafic igneous rocks such as basalt and peridotite.
Common olivine is very widespread; peridot comes from China, Myanmar, Brazil, Australia, South Africa, Noroway, and from Hawaii and Arizona.
Peridot beads were made by the Egyptians as early as 1580-1350 BC. Peridot was considered a symbol of the Sun from ancient times to the Middle Ages, and an early Greek manuscript informs us that it confers royal dignity on its bearer. To be proteced from evil spirits, the owner of peridot should have it pierced, strung on the hair of a donkey, and tied around the left arm--according to the 11th century French bishop Marbodius, at least. Strangely for such an ancient gem, there is relatively little other esoteric lore associated with peridot. In all likelihood, this is because it was grouped with other green stones as Pliny's smaragdus stone. Some historians believe that Cleopatra’s famous emerald collection might actually have been peridot. People in medieval times continued to confuse peridot with emerald. For centuries, people believed the fabulous 200-ct. gems adorning the shrine of the Three Holy Kings in Germany’s Cologne Cathedral were emeralds. They are, in fact, peridots.