Turquoise was one of the first gemstones to be mined and has long been prized for its intense opaque colour, which varies from sky-blue to green. A pure blue colour is rare, most pieces contain matrix, i.e., veins which may be brown, grey or black. Reconstituted turquoise has an even blue colour but is made from powdered turquoise mixed with resins. The popular sky blue colour changes into a dull green when the stone is subjected to intense heat. Light, perspiration, oils, cosmetics and a loss of natural water content can also make a stone turn green, which explains why the turquoise in older pieces of jewellery is often green in colour. Turquoise has been enjoying a revival recently with Native American style turquoise jewellery featured in all the fashion magazines. Turquiose is the birthstone for December.