Opal, along with tourmaline, is the official birthstone of October and it is the product of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.”
The showers soaked deep into ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward.
During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.
Australia produces around 97% of the world's opal. 90% is called 'light opal' or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields.
Opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal.