August: Month of Peridot

October 21, 2002
Burke Museum
Peridot comes from the volcanic rock that is composed primarily of olivine.

Photo: “Peridotite mantle…” by James St. John is licensed under CC BY-ND 2.0.

Peridot is a type of olivine.

Photo: “Peridot” by Jeff Schultz is licensed under CC BY-ND 2.0.

History of peridot

The Arabic word "faridat," meaning gem, is where the name for the gem peridot comes from. Peridot's stunning beauty and bright color caused the ancient Egyptians to call it "the gem of the sun." It was believed that peridot glowed with its own inner light even after sunset, and that miners used to locate the stones at night to retrieve them during the day.

Science of peridot

Peridot is type of olivine, and comes in various shades of green, from light to a brilliant olive green. Olivine contains two types of minerals: fayalite and forsterite. Fayalite, with the mineral formula Fe2SiO4, contains large quantities of iron. Forsterite contains magnesium, and has a formula of Mg2SiO4. Olivine's formula is (Mg, Fe)2SiO4, showing the substitution of magnesium and iron. Peridot is the transparent form of forsterite.

Group: Olivine
Class: Silicates
Subclass: Nesosilicates
Chemistry: (Mg, Fe)2SiO4, magnesium iron silicate

An alternative birthstone for August is the gem variety of jade. The green color in jade is from trace amounts of chromium in the mineral.

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