Opal

The mineraloid opal is amorphous SiO<sub>2</sub>·nH<sub>2</sub>O; hydrated silicon dioxide, the water content sometimes being as high as 20% but is usually between three and ten percent. Opal ranges from colorless through white, milky blue, gray, red, yellow, green, brown and black. Common opal is truly amorphous, but precious opal does have a structural element. The word opal comes from the Sanskrit upala, the Greek opallios, and the Latin opalus, meaning "precious stone." Opals are also Australia's National gemstone.