Copper, again giving us bright lively greens, mineralised this time as a chloride (as table salt is to sodium) named for the type locality, a mine in Cornwall, where it is a secondary mineral. These are formed by the alteration of the primary ores by mineralised brines passing through the rock after deposition, though it has also been found at black smoker vents where the primary dark sulphides streaming out react with the cold sea water and in copper slags in abandoned foundry sites . It was first described in 1865 when mineralogists were scouring the growing mines around the world as the empires peaked in quest of new additions to the known repertory.
It is similar to another mineral called Atacamite (see http://bit.ly/2nPqOoI), which shares the same chemical composition but crystallises in a different shaped structure, a property known in mineralogy as polymorphism. Alongside the United Kingdom it has also been found in Scotland, Ireland, Germany, Greece, South Africa, the United States, and along the Mid-Atlantic Ridge. The 6.3 x 3.4 x 3.1 cm in the photos was mined in Cornwall.
Image credit: Rob Lavinski/iRocks.com