What is a Turquoise Gemstone

Article is from Geology.com

What is Turquoise?

Turquoise is an opaque mineral that occurs in beautiful hues of blue, blue-green, and yellow-green. It has been treasured as a gemstone for thousands of years. Isolated from one another, the ancient people of Africa, Asia, South America and North America independently made turquoise one of their preferred materials for producing gemstones, inlay, and small sculptures.

Chemically, turquoise is a hydrous phosphate of copper and aluminum (CuAl₆(PO₄)₄(OH)₈·5H₂). 

Its only important use is in the manufacture of jewelry and ornamental objects. However, in that use it is extremely popular - so popular that the English language uses the word "turquoise" as the name of a blue-green color that matches the stone. Very few minerals have a color that is so well known, so characteristic and impressive that the name of the mineral becomes so commonly used. Only three other minerals - gold, silver, and copper - have a color that is used in common language more than turquoise.

Turquoise Colors

Blue minerals are rare, and that is why turquoise captures attention in the gemstone market. The most desirable color of turquoise is a sky blue or robin's-egg blue. Some people inappropriately describe the color as "Persian blue" after the famous high-quality material mined in the area that is now known as Iraq. Using a geographic name with a gem material should only be done when the material was mined in that locality.

After blue, blue-green stones are preferred, with yellowish green material being less desirable. Departure from a nice blue color is caused by small amounts of iron substituting for aluminum in the turquoise structure. The iron imparts a green tint to the turquoise in proportion to its abundance. Turquoise, especially the more porous varieties, can discolor with exposure to prolonged sunlight, heat, cosmetics, perspiration, and body oil.

Some turquoise contains inclusions of its host rock (known as matrix) that appear as black or brown spider-webbing or patches within the material. Many cutters try to produce stones that exclude the matrix, but sometimes it is so uniformly or finely distributed through the stone that it cannot be avoided. Some people who purchase turquoise jewelry enjoy seeing the matrix within the stone, but as a general rule, turquoise with heavy matrix is less desirable.