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Rockhounding Washington Green Opal From a Diatomaceous Earth Mine (YouTube) This location is a fun little stop off I-90 north of Vantage, WA. This is an active mining operation so going during business hours may be an issue so it’s best to stick to the weekends, and after 5pm.

Opal (Wikipedia) Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.