Opal is hardened balls of silica gel that form with water in between them.

Chemical Formula: SiO2-nH2O
Class: Silicates, tectosilicates
Hardness: 5-6
Coloration: colorless, white yellow to orange, rose, black, red, of dark blue.

How to identify it/Unique Characteristics or Properties: Opal is hardened balls of silica gel that form with water in between them. This creates the shimmer that opals are prized for. Without very even alignment of the silica balls, the shimmer does not happen. Most opals are not gem quality as the silica arrangement is poor and so they are dull. Different impurities give opals the wide range of colors that they comes in. Opals never grows large crystals, but the smaller the crystal size, the higher the quality of gem.

Where to find it in…

The Northwest: n/a
Other: Mexico, Slovakia, Australia.
Past Uses: Opals are used in jewelry and prized for their beautiy. Low grade opals are mined for abrasives.
How it was Made: Opals form in cavities in other rocks where water with a high silica content gathers and slowly filters out, leaving the silica behind in ordered patterns.
Interesting Facts: If opals dry out too much they can crack and even lose their color, as much of an opal is still water inside. Some fossilized wood, especially in Australia, can also be made from opal, as in the picture below.