Opals
Finished or in the rough — Straight "From the Mine to You"
Opal is a gemstone made of hydrated silica, with the chemical formula SiO₂·nH₂O. Unlike most gemstones, opal contains water within its structure. GIA says opal can contain up to about 20% water and has a Mohs hardness of approximately 5–6.5. Opal is an October birthstone, along with tourmaline.
Origin
Important opal sources today include Australia, Ethiopia, Mexico and Brazil, with additional deposits in places such as Honduras, Indonesia, Madagascar, Peru, Turkey and the United States.
How to test the "Realness" of an Opal
With a loupe or magnifying glass, examine the stone from the side and back, not just from the top. An opal doublet consists of a thin layer of opal attached to a backing. A triplet typically has an opal layer sandwiched between a dark backing and a transparent dome of material such as quartz or glass. From the side, you may be able to see these distinct layers.
Interesting Facts about Opals
No two natural opals have exactly the same arrangement of colors and patterns. Opals can also be transparent, translucent or opaque, and not every opal displays rainbow play-of-color. Important quality considerations include the brightness and range of play-of-color, pattern, background color and clarity.