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The smooth egg shape of this specimen isn’t natural, but the rainbow-colored opal vein inside is. This specimen was mined in 1985 in Oregon. The brown areas are rhyolite, a volcanic, igneous rock. Opals are made from silica (the same stuff as sand or quartz), but are infused with water molecules. The arrangement of the silica diffracts light, causing opal’s multicolored sheen.