Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. This Diamond shaped piece of opal is set in a Silver mount and measures 2.6cm x 1.5cm approx. It is suspended from an 18 Inch 925 Silver chain. Opal is the birthstone for the month of October, along with Pink Tourmaline. It is also the stone given to celebrate the 14th year of marriage.