The mineral turquoise is a hydrated copper-aluminium phosphate, formed by precipitation from aqueous solutions in a host rock rich in copper and aluminium. It is amorphous (non- or crypto-crystalline) and has a characteristic blue green colour. It is of rare occurrence and since ancient cultures has always been highly valued as a gem for use in jewellery as well as religious and artistic crafts etc. This picture is of an unworked pebble from a locality in North Carolina, USA, well known as source of turquoise and other semi-precious stones. Actual width is 50 mm.