Opal

A play of colour from microscopic silica spheres.

Mohs hardness: 5.5–6.5
Crystal system: Amorphous
Composition: SiO₂·nH₂O
Colours: White, Black, Boulder
Origins: Australia (Lightning Ridge, Coober Pedy), Ethiopia (Welo), Mexico
Birthstone: October

Opal contains 3–21% water and is built from stacked silica spheres that diffract light into spectral 'play-of-colour'. Black opal from Lightning Ridge - with a dark body that intensifies the fire - is the most valuable variety.

Ethiopian Welo opal, on the market since 2008, is hydrophane: it temporarily absorbs water and changes appearance.

Pliny called it the loveliest of gems; later European folklore wrongly cast it as unlucky after a Walter Scott novel.

Treatments

What to know

Smoke / sugar-acid treatment - darkens body colour (disclosed)
Polymer impregnation for stabilisation

Care & handling

How to wear it

Avoid heat, ultrasonic, steam and harsh chemicals
Never store in a safe deposit box for long periods - can craze if dried out
Wipe with damp cloth; avoid prolonged immersion for hydrophane material

Jewellery use

Setting the gemstone

Pendants, earrings, brooches and protected ring settings. Doublets and triplets extend wearability of thin material.

Famous examples

· Olympic Australis (17,000 ct, rough)
· Aurora Australis (180 ct, black)