Peridot and Olivine in Volcanic Terrain

Olivine-rich basalt (picrite) is the hunting ground for gem-quality peridot. Look for rounded, olive-green crystals weathering out of dark basalt — the surrounding rock erodes faster than the harder olivine. Xenoliths (mantle fragments brought up by eruptions) are the best source: look for rounded greenish nodules in basalt flows and cinder cones. Hawaii, Arizona's San Carlos Reservation (the world's largest peridot deposit), and New Mexico's Kilbourne Hole maar are premier US localities. Fresh, unweathered material shows clear glassy luster; rusty-brown oxidation rims indicate iddingsite alteration and reduced gem quality.