Opals are formed of tiny spheres of silica that are stacked together in an orderly grid-like structure, which refracts light and forms the precious mineral that we call opal. Opals are considered mineraloids because they lack a defined crystal structure. They also have a water content ranging from 3-20%.
How are Australian opals formed?
Around 100 million years ago, much of Australia was covered by a vast inland sea. As the sea slowly evaporated, it left behind silica deposits. Over the millions of years that followed, groundwater carried those solutions through the sandstone.
How are Ethiopian opals formed?
Ethiopian opals are formed in volcanic rocks when silica-rich fluids interact with volcanic ash and rhyolite. Most Ethiopian opals are hydrophane, which means they can absorb water like a sponge. When placed in water, they may temporarily become transparent or change color.
How long does it take an opal to form?
30 million years ago, when lots of silica was released into a solution that filled cracks in the rocks, layers in clay, and even some fossils, the very beginnings of the creation of opal started. It’s believed that it takes about 5 to 6 million years for a 1cm opal to mature.