Opal is hardened silica gel that still contains some water. Precious opal shows play-of-colour: patches of pure colour that shift as the stone moves. It is caused by light diffracting on tiny spheres of silica, stacked in a regular pattern.
The main types
- White or light opal: a pale body with play-of-colour, mostly from South Australia.
- Black opal: a dark body that makes the colours stand out. It comes from Lightning Ridge in Australia and is the most valuable opal.
- Boulder opal: thin seams of opal cut together with the brown ironstone they formed in, from Queensland.
- Fire opal: transparent orange to red, often without play-of-colour, mostly from Mexico.
- Ethiopian opal: bright and often very lively. Much of it is hydrophane: it absorbs water and can temporarily turn transparent or change colour when wet.
Solid, doublet or triplet
A solid opal is opal all the way through. A doublet is a thin slice of opal glued onto a dark backing. A triplet adds a clear cap on top. Doublets and triplets cost far less and must be sold as such. Look at the stone from the side: a straight glue line gives them away. Do not soak them, because water can get into the layers.
Wearing it
Opal has a hardness of 5.5 to 6.5 on the Mohs scale and is sensitive to knocks. It is at its best in pendants, earrings and brooches. In a ring it needs a protective setting, such as a bezel, and should come off for manual work. See which stones can be worn every day.
Care
Avoid heat, very dry conditions and sudden changes of temperature, which can cause fine cracks. No ultrasonic cleaner. A damp soft cloth is enough.
Opal is a birthstone for October. See opal jewellery.