The modern August birthstone, peridot has been prized as a jewelry stone since ancient times. Always green in color but with considerable variations, a peridot’s particular shade depends on its source.
Peridots belong to the forsterite-fayalite solid-solution series, which forms part of the olivine group of minerals. The term peridot refers to green, gem-quality olivine. Forsterite is the magnesium (Mg)-dominant end, while fayalite is the iron (Fe)-dominant end. As idiochromatic gems, peridots get their characteristic green color from iron, an essential element of their chemical structure. In peridot, ferrous iron (Fe2+) creates green, while ferric iron (Fe3+) creates yellow. Traces of chromium (Cr) in peridots don’t cause hue but may make the green color brighter.
An iron content of just 12 to 15% in olivines creates the ideal peridot color, whereas higher levels create a “muddier” brown color. Thus, olivines closer in composition to the forsterite end tend to be greener. In contrast, those closer to iron-dominant fayalite tend to be yellower and browner. Therefore, most peridots are closer to the forsterite end.
Peridotite
Olivines (those from Earth, not from outer space) form in the Earth’s mantle. Peridotite, an igneous rock made primarily of olivine and pyroxene, makes up most of the upper mantle.
Fayalite
Brown olivine gems, rich in iron, can be very beautiful, especially when the color runs more golden than brown. Although very rarely found in nature, fayalite commonly occurs in iron slag produced by metallurgical processing.
Timothy Mountain, east-northeast of Lac La Hache produces bombs from which excellent dark green stones weighing as much as 10 carats have been obtained. Lightning Peak in the Monashee Mountains produces peridot grains large enough to cut small gems from.
