Turquoise is a copper phosphate comprising aggregates of tiny crystals. Deposits usually form in iron-rich limonite or sandstone. Limonite creates dark brown markings in turquoise, while sandstone creates tan markings.
These markings, referred to as ‘matrix’, are remnants of the host rock and can resemble splotches or veins.
While turquoise is often associated with a light to mid-blue colour, it can also be found in greyish shades as well as greenish blue through to green – though blue hues are more popular.
The most prized turquoise displays mid- blue colour with no matrix, usually cut as cabochons and inlays. Matrix turquoise is appreciated when it has fine, delicate, web-like patterns across the surface of the gemstone, when it is often called ‘spiderweb turquoise’.
Usually a fairly porous mineral, polished turquoise has a waxy lustre – though less porous examples are tougher and can have a better lustre. It rates 5–6 on Mohs’ hardness scale.
The porosity of turquoise makes it difficult to handle as a gemstone. Constant exposure to liquids and grime – including perspiration, cosmetic sprays, fats, oils and greases – can stain or discolour the surface, causing it to turn a less attractive greenish colour.
Consequently, the common, but non- permanent, practice of waxing turquoise is an accepted treatment that need not be disclosed to customers.
However, waxing is not a long-term preventative of discolouration and the turquoise trade has become blighted by a multitude of other treatments to seal the porous surface. Some of these treatments are very difficult to identify.
Sealing the pores with paraffin, sodium silicate, organic polymers and sundry epoxy resins has taken place many times in recent years. Some are of a more permanent nature, but the use of manufactured compounds to seal the pores is considered less acceptable and such treatments should be disclosed.
Turquoise offcuts and other broken pieces that have been powdered up and mixed with resins to produce so called ‘reconstituted turquoise’, is another less acceptable treatment that requires disclosure.
Synthetic turquoise – that is, imitants that are neither the chemical nor physical equivalent of the natural material – can be convincing to the unaided eye.
The many imitants include dyed forms of other minerals, sometimes with a lookalike matrix effect. One such product, often manufactured in China, is ‘turquentine’ – a non-gemmological term for howlite or magnesite that has been artificially coloured blue.
Some of the best quality turquoise has come from Iran, previously known as Persia, and thus some refer to the best- coloured turquoise as ‘Persian blue’ turquoise.
The southwest of the US is well-known for the turquoise mined by Native Americans and used in jewellery and as amulets; this region remains one of the largest consumer markets for turquoise. The Aztec civilisation – located in modern-day Mexico – also highly valued turquoise, considering it to be sacred.
China’s Hubei Province is a new source, while the historic Egyptian mines in the Sinai Peninsula have been depleted.
Closer to home, a turquoise mine worked in the Northern Territory was closed in the mid 1990s, and the gemstone was also briefly mined in Victoria and regions of NSW including Narooma and Bodalla at the beginning of the 20th Century.
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