The only significant precious-opal mine in the United States open to the public. The opal formed in the cavities of a rhyolite lava flow, and it comes out in thin layers that are usually made into triplets to be worn. There are two ways in: dig the pit yourself on scheduled days in summer, or pick through the mine-run rock at the shop. This is high desert at 6,000 feet on the Montana line, so it is a summer-only proposition. PUBLIC fee dig, Spencer (Clark County).
Precious opal
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Other places on the map within a couple of hours.
