Peridot, (Mg, Fe)₂SiO₄, is the gem quality version of the mineral olivine. It’s the gemstone for those born in August and for 15th wedding anniversaries.

Fossil Ammonite with peridot, cultured pearls, and gold-filled. Item FAGFN103
Background of natural olivine crystals.

Peridot comes in a relatively narrow range of colors compared to most gemstones. That’s because the color is due to the amount of iron in the mix and not due to chemical impurities or distortions of the crystal structure. The color range goes from yellowish greens through brownish greens. Peridot crystals tend to have consistent color throughout (contrast this with a gem like amethyst which often goes from colorless through various saturations of purple in the same crystal).

From a gemology perspective, one of the cool features of peridot can only be seen with a microscope; there are often little inclusions called “lily pads” that are very distinctive and help identify the gem.

Olivine is often found in iron-rich rocks and can be seen filling the nodules left when gas escapes from molten lava. Been to Hawaii? If you go again, look around when you see the blocky types of basalt (cooled lava) with lots of holes – you may spot some olivine! It has also been found in meteorites.

There is a major deposit of gem quality peridot on the San Carlos Apache reservation in Arizona. Other notable locations for peridot include China, Pakistan, Vietnam, Myanmar, Egypt, and Tanzania.

Peridot has hardness of 6.5 to 7 on the Mohs scale. It has fair to good toughness. Warm, soapy water is the best method for cleaning peridot. Ultrasonic and steam cleaners are not recommended.

Sources:
http://www.gia.edu/peridot
https://en.wikipedia.org/wiki/Peridot