December has three birthstones – turquoise, tanzanite and zircon. Turquoise was on the original birthstone list published by the American National Retail Jeweler’s Association (now Jewelers of America) in 1912. In 1952, tanzanite was added and in 2002, zircon. All three stones are blue, although zircon can be found in many other colours. Turquoise, though, is an opaque gemstone, unlike zircon and tanzanite.
Turquoise is one of the oldest stones found to have been used for ornamental purpose. Examples have been found in discoveries from the Egyptian, Persian and Mesopotamian civilisations, across to China and other parts of Asia, and onto North and South America. Turquoise was mined in the Sinai Pininsula from around 3100BC but turquoise beads were found much earlier than that. Mines in Persia have been operating from at least 4000BC. China too was also a source of turquoise in ancient times. Turquoise mining in the Americas appears to date from around 500BC.
The name ‘turquoise’ arose in the 17th century, derived from the French expression ‘pierre turque’ (‘Turkish stone’) as it arrived in Europe via Turkey, mainly from the Persian mines.
Typtically found in arid regions, turquoise is formed when acidic watery solutions seep into rocks and fill crevices. It is often interwoven within a matrix of the host rock. This matrix can be hard to spot, like a spider web in some instances or it can be a striking feature of the stone. Turquoise is also very porous and so can darken and turn greener due to perspiration and access to water.
Colour can vary from sky blue to a greenish blue, often as a result of where it is mined. Persian turquoise is a sky blue, usually a uniform colour, while turquoise from the US mines can have a greenish colour as it contains iron.