A pale-green or yellow-green gemstone of the olivine family. (Gorgonzola tells me it's "specifically gem-quality Forsterite (Mg2SiO4).") Chrysolite, given as the same thing is Webster 1913's writeup, is really a generic term for gems of the same color, including chrysoberyl. Some peridots have wound up labeled as emeralds if they had a deep enough color, and historically peridot was considered to be the same thing as topaz and the Red Sea island where it was first mined was even called Topazios (it's now Zabargad), but modern mineralogy sorted out their different compositions.

Currently almost 80% of peridot on the market comes from Arizona, but many other locations produce small amounts. Peridot itself is not rare, but large crystals of the stone are.