Gem Minerals
Opal
What it is
Not a true crystal but a mineraloid — a hardened silica gel with water trapped between microscopic silica spheres. When these spheres are uniform in size and stacked in orderly layers, they diffract light into the rainbow play-of-color that makes opal unique among gems.
How it forms
Forms when silica-rich water percolates through rock and deposits in cavities over millions of years. Australia's outback produces 95% of the world's precious opal, formed in the weathering zone of ancient sea beds.
SiO₂·nH₂O (Amorphous)
