Opal semiprecious gemstone index and mineral information

The word opal derives from the Indian word “stone.” There are three categories for opal; precious opals, yellow fire opals, and common opals. The special characteristics of opal are their play of color, a display of rainbow like hues which changes with the angle of observation.

Opal is a form of silica but different from quartz insomuch as it contains water in varying degrees.  It is classed as a solidified jelly or gel.  There is a wide range of opals but the most important ones are:  Precious opal, which is the most expensive and is divided into three colors; black, blue, green or grey; also intense colors as red and violet which are the most expensive in the market; Light opal, in which the body color is cream to white; and Clear or Water opal, which is clear with a good play of color.   Fire opal is transparent or translucent orange to red, generally without a play of color.  The principal sources of fire opal are Mexico and Honduras where it is found in trachyte, as well as in Brazil, Guatemala, the United States and Western Australia; and Jelly opal, or “potch”, which is common opal that has no color and is generally discarded.

Opal is deposited at low temperatures from silica-bearing, circulating waters.  It is found as nodules, stalactitic masses, veinlets, and encrustations in most kinds of rocks. Precious opals can form only in undisturbed space within another rock that is capable of holding a clean solution of silica from which water is slowly removed over a long period; perhaps thousands of years.  All precious opal is relatively young in geological terms, since precious opal cannot withstand the heat and pressure of burial and metamorphism.

Until late in the 19th century, the primary source of precious opal, including that used by the Romans, was in present-day Slovakia, where opal occurred in andesite.  Today, the main producer of precious opal is Australia, where it was discovered in 1887.  Here it is found in sedimentary rocks such as sandstone and ironstone.  The Lightning Ridge field in New South Wales, Australia, produces the rare and prized black opal, with a very dark gray or blue to black body-color and a superb color play.  Until its discovery in 1903, black opal was virtually unknown.  It is ten times more expensive than light opal and consequently various treatments are applied to the latter to give the appearance of black.  Myth and legend describe opal as “Cupid’s Stone of hope and love.”  It was held in the greatest esteem as it contained the red of the ruby, the blue of the sapphire, the green of emerald, the gold of topaz, and the purple of the amethyst, thus it carried all the properties of all these stones.  Only emerald was held in higher esteem.  Many people over the centuries believed this gem to bring good luck. 

Opal can be confused with ammonite, labradorite, mother-of-pearl, and moonstone.  Several good imitations made from glass or plastic are also known.

Birthstone: Opal is one of the twelve gemstones of the ceremonial breastplate worn by the high priest Aaron, representing the twelve tribes of Israel as described in the Old Testament book of Exodus, and is also known as the month of October’s birthstone meaning, hope.

Mohs’ hardness: 5.5 – 6.5