Continuing on my geology “kick,” here’s a beautiful sample of Opal on display at the Houston Museum of Natural Science:
This particular sample was found in the Opalville Mine in Queensland, Australia.
Opal is a mineraloid (like a mineral, but without a crystal structure) related to quartz — it’s a hydrated silica that forms in fissures of all sorts of rocks, but 97% of opals are mined in Australia. Lesser amounts of opal are produced by the U.S., the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia. Opaline minerals have also been found on Mars, an indicator of once-wetter times on that planet.
From the specimen’s museum placard:
Discovered in 1989, this is one of the largest (2,765 carats) and finest quality boulder opals ever mined. This opal is unique not only for its size but also for its quality — every color of the spectrum is visible which is extremely rare, even in the finest opals.