Faustite

Faustite was discovered in the Copper King Mine in Eureka County, Nevada.  It was named in 1953 after George Tobias Faust, a U.S. Geological Survey employee. 

This is a rare stone that has shown up in only a few deposits.  It is typically found as a vein or in spot formations within chert, shale and limestone.   It also tends to be mixed with a local type of clay called Montmorillonite, which makes this stone quite brittle.