Peridots have been prized as jewelry stones since ancient times. Always green in color but with considerable variations, their particular shades depend on their locality of origin.
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. While traces of chromium (Cr) in peridots don’t cause hue, they may make the green color brighter.
An iron content of just 12 to 15% in olivines creates the ideal peridot color. 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.