Opals are a form of silica in a non-crystalline structure. It generally has a water content of around 6 to 10%, but it can range as low as 3% and up to 21% by weight.
Australia produces around 97% of the worlds supply, chiefly mined at Coober Pedy in South Australia and Lightning Ridge in New South Wales.
Precious opals diffract light due to their internal structure,
spheres of SiO2 (silicon dioxide) in planes closely packed
together. The colours presented by an opal depend upon the conditions
where it was formed. They range from being opaque to semi-transparent
and display all the colours of the rainbow as well as black, brown,
slate, pink and grey.
Greens and whites are the most common colours, a black background with red hues above are the least commonly found.
A Potch opal (common opal) has a hazy, milky appearance (opalescence) and does not display the bright colours seen in precious opals. The fiery bright colours displayed by precious opals is known as the ‘play of colour’.
The vast majority of these gems are cut and polished into cabochons for use in jewellery.
Rings, pendants, brooches, earrings and cuff links are commonly set in either silver or gold. The colours of these stones suit both precious metals.
Solid opals are stones that have been cut and polished from a single piece of opal. Some opals are too thin to be used in jewellery ‘as is’ and are fitted to a dark backing layer of a common (potch) opal or ironstone. A dark background highlights the play of colour in the top stone giving more depth to the ‘doublet’. Doublets can produce a similar effect to that of a black or boulder opal, but at a much lower price.
A triplet is simply a doublet as described above with an extra top layer of clear quartz or plastic. Normally it would be domed (as in the shape of a cabochon) and this third layer gives some protection to the opal below as well as acting like a magnifier. Triplets are not considered to be precious opals.
Common opals don’t display the play of colour of precious opals. Milk opals (milky bluish/greenish), resin opals (yellow/brown) and wood opals are examples.
Fire opals display the warm colours of red, orange, yellow but do not
usually show a play of colour, and vary between translucent and
transparent, although some stones can exhibit a flash of bright green.
Many fire opals come from Mexico.
Peruvian opals are semi-opaque to opaque and typically blue to blue green as their primary hue.
Black opals with a dark background of blue/black or dark grey colour are mainly sourced from Lightning Ridge. Coober Pedy is the largest opal producer in the world supplying around 70% of the total.
Opals come in at 6.5 on the Mohs scale, around that of glass. This means they can be easily scratched, so remove it before gardening or performing other physical tasks.
Solid opals can be cleaned with warm water and a mild detergent, they will not be damaged by water. Doublets and triplets should be wiped with a damp cloth but never immersed in water as the layers can separate, but this will take a prolonged exposure.
Ultrasonic cleaners can cause cracking in a solid opal and water to penetrate a doublet or triplet and so should never be used.
Zero humidity safes are NOT recommended for opal storage. For
prolonged storage place your opal in cotton wool with a couple of drops
of water in a sealed plastic bag. The opal will not absorb the water,
but it will prevent moisture leaving the stone.