O C T O B E R

OPAL

Opal is a hardened silica gel, usually containing 5–10% water. It is therefore non-crystaline, unlike most other gemstones, and may eventually dry out and crack. There are two varieties; precious opal, which shows flashes of colour (iridescence), depending on the angle of viewing; and common or “potch” opal, which is often opaque and displays no iridescence. The iridescence of precious opal is caused by the way the structure, a regular arrangement of tiny silica spheres, diffracts light – the larger the spheres, the gbreater the range of colours. Precious opal occurs in a number of colour varieties.