Opal is a form of silicon dioxide in which many colours, caused by reflections from microcrystalline silica spheres, may be seen when it is turned in the light. Opal is divided into two types – black or white – according to the background colour of the stone. It’s value is determined by the brilliance of the colour, the absence of dead spots and flawlessness. Black opal is considered the finest and those with an abundance of red are the most valuable. Opals which are strong in green and blue are not as rare.
Opal was thought to be a bad omen yet to be a symbol of fidelity and to have the ability to cure diseases of the eye.
Opals tend to dry and crack and benefit from an occasional coating with olive oil.