Opal is primarily composed of a solution of silicon dioxide and water. 
As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into tracks and voids, caused by natural faults or decomposing fossils. Once the water evaporates it leaves behind a silica deposit. 

Where Opals can be found?
Most of the precious Opal deposits that have been discovered are in Australia. 
The mines of Australia produce at least 90% of the worlds precious opal.
Famous mining areas in Australia include: Koroit, Coober Pedy, 
Lightning Ridge, Mintabie, Andamooka, Yowah, Jundah and Quilpie. 

What is the meaning of Opal? 
Opal has always been associated with love and passion, as well as desire and eroticism. 
It is a seductive stone that intensifies emotional states and releases inhibitions. It can also act
as an emotional stabilizer. Wearing an Opal is said to bring about loyalty and faithfulness. 

Opal is known for its unique display of flashing rainbow colors called play-of-color.