Fire Opals

Opal is an amorphous material, since it has no defined lattice structure. It crystallises in various ways, in compact masses, botryoidal, stalactitic, globular and others. it is hard and light, it does not cleave but is very brittle.

Its appearance and colouring vary according to the variety. Precious opal has a grey, blue or black background with brilliant plays of colour, generally red, blue and yellow. Fire opal has reflections ranging from yellow to red. Hydrophane is whitish.

It shows evident fluorescence when exposed to ultraviolet rays and is insoluble in acids, except in hydrofluoric acid. If heated it loses water molecules and can turn into quartz.

It is of sedimentary origin, and forms mostly in silica-rich waters, especially around hot springs.

It is used above all in goldsmiths' work.