Opal is found around the world (Brazil, Mexico, Honduras and the western US) however Australia produces 95% of the world's precious opal and it is our official national gemstone. (see Google Arts and Culture - Australian opals).

How is an opal formed?

Opal is comprised of tiny silica spheres, formed when silica-rich water seeps into deep cracks and voids in the Earth's crust. It is separated into two groups, common opal (also known as potch) and precious opal (also known as noble opal). ... A cross section of opal in its host rock.

What color opal is most valuable?

The 'dominant colour' in an opal can affect its value, with red commanding the highest cost, followed by orange, yellow, green, blue, indigo and violet. However, do not forget, a blue-green brilliant stone will be more valuable than a dull red.