HOW OPAL IS MADE...
In the formation of opal, silicon dioxide mixes with water. From the pressure of flowing water, silica from sandstone moves into cracks and voids caused by natural faults or decaying fossils. When water evaporates, it leaves behind a silica deposit. Over very long periods of time, this cycle repeats, and eventually the formation of opal occurs.
HOW LONG DOES IT TAKE FOR OPAL TO FORM?
This is a long process. It takes around 5 to 6 million years for a 1 cm opal to mature.