October-Opal

Colour	Variable
Formula	SiO2 with water attached (up to 10%)
Group	Oxides
Uses	Jewellery
Origins	Often found in sedimentary rocks
Locations	Romania, USA, Australia, Devon and Cornwall UK

Opal is a form of quartz and has no crystal structure. It's colour can be altered by the heat of a hand.

It is made from layers of silica and the size between the layers determines it's colour (this can vary from white to black). The more colourful the stone the more valuable it is.

It is fairly soft and often transparent. When it loses water it may turn into chalcedony or quartz.