The Spencer opal mine is situated 65 miles north of Idaho Falls at the Interstate 15 and 70 miles north of the Yellowstone national park. The mine was discovered 1948 by two hunters and filed as mining-claim in 1952 . In the year 1964 the family Stetler acquired the claim. Rockhounds are invited to dig against paying small fee in the so called “mini mine” since 1968.

The precious opals are occuring  here as fillings in thunderggs.Typically for Spencer precious Opal is the occurrence in thin layers between not opalizing common opal. This is explained by the mine owners with the rhythmic setting of silica-gel causes by geysers* . Due to this formation the Idaho opals are used rarely to produce solid cabochons, rather as triplets.

* However this formation of opal occurs also in other places of the world - among others in australian specimens - without designating that its formation is connected with volcanic activity. The sedimentation of opal in thundereggs is following rather the principles of the “self-organization” - similar as the agate-formation - which does  not postulate an outside rhythm but rather an inside rhythm.

Further information are given from the mine owners on their website http://www.spenceropalmines.com/.