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Opal is the birthstone for the month of October, along with pink thourmaline. It is also the stone given to celebrate the 14th year of marriage.The name opal is thought to be derived from the Sanskrit upala, meaning “precious stone,” and later the Greek derivative “Opallios,” meaning “to see a change of color.”
Opal is a delicate and soft stone, rating a 5.5 to 6.5 on the hardness scale. It is usually milky and translucent. Opal is a hardened silica gel containing 5-20% water. Some opals may crack if allowed to dry out too rapidly after being mined. Opals may be somewhat porous, in which case it is dangerous to immerse it in liquids other than water. Opal is amorphous, meaning, it has no crystal structure. The only other major amorphous gemstone is amber. Good quality opals are transparent, not milky.
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence), common opal does not.The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque and the background color may be white, black, or nearly any color of the visual spectrum. Black opal is considered to be the rarest, whereas white, gray, and green are the most common.