The science: gem-quality opal is one of the most spectacular gemstones. A single stone can flash every color of the spectrum with an intensity and quality of color that can surpass the “fire” of diamond. The best opals can command prices per carat that rival expensive diamonds, rubies and emeralds. Opal is one of the most popular gems.
Opal is a hydrous silicon dioxide (SiO2.nH2O). It is amorphous, without a crystalline structure, and without a definite chemical composition. Therefore, it is a “mineraloid” rather than a “mineral.”