Andean Opals are exported from Peru since the 1980ies. They originate from the copper mining area of Acari in the Arequipa province. Their colour is rarely purely blue, usually greenish blue to green. The translucence of the stones ranges from transparent to to opaque. In particular, the intensive green, transparent stones can change colour in the course of time – from the fresh green into a muddy-green. Also, these stones lose transparency. This process is caused by dehydration, as well as by the chemical change of the colouring copper minerals in the opal.