Where and how does fire opal form?
The area in Mexico called Queretaro, which lies about 200 kilometers north-west of Mexico City, is the world’s only commercially significant source of fire opal. The opals occur in cavities and fractures within rhyolitic lava flows. Some geologists have speculated that these rich-orange to red stones might have been associated with the high temperatures involved in the formation of these igneous rocks, but recent research shows that this is not the case. Rhyolite (a silica-rich volcanic rock) crystallizes at about 800°C (1472°F), which is pretty hot in anybody’s book, even though basalts are even hotter by about 300°C (572°F). However, a recent scientific study established that the fire opal from the Queretaro area formed at only 160°C (320°F), which might sound like a lot if you’re talking about bath water but is quite cool by geologic standards.
