Opal

opalGemology

Translucent mineral consisting of hydrated silica of variable color. Precious opal set in jewelry has a rich iridescence and remarkable play of changing colors

ColorWhite, black;, gray and others with play of colors

Family:  Hydrous form of silica

Chemical CompositionSiO2. n H2O

Crystal SystemAmorphous

Hardness (Mohs Scale)5,5 - 6,5

Specific Gravity:  1,90 ±0,30

Refractive Index1,44 ±0,03

BirefringenceNone

Optical Character:  Singly refractive

Myths

The play of colors seen in opal is caused by the diffraction of light from regularly arranged transparent spherical particles of cristobalite, an amorphous silica, and the voids between these particles. These microscopic particles have a diameter of approximately 1/10,000 of s millimeter.  The angle of the light diffracted varies with the wavelength and thus only pure spectral colors can be seen.  The name opal comes from the Latin “opalus”, originates from the Sanskrit “upala” meaning precious stone.

Archaeologist Louis Leakey found six-thousand year old opal adornments in a cave in Kenya.  The ancient Greeks believed opal could give to the wearer the power of foresight.  The Romans revered opal as the symbol of hope and purity and believed it could protect from disease. Eastern people regarded it as the symbol of truth. The Ancient Arabs believed it came from heaven, and that it acquired the play of color from flashes of lightning.  In the Middle Ages, opal was thought to be beneficial for eyesight, some people even believed it could render the wearer invisible.


French Emperor Napoleon gave to his wife Josephine an opal called “The Burning of Troy” because of its strong play of colors.  In the nineteenth century, the opal was described as an unlucky stone in the novel “Ann of Geirsteir” by Walter Scott which temporarily minimized the opal’s popularity. Public figures such as Queen Victoria as well as the French actress Sarah Bernhardt eventually reestablished the gem’s popularity.

Opal is classified in three primary types according to its body color supporting the play of colors. These types are black, white and transparent.  Transparent opal can have body colors such as colorless, yellow, brown, red, or orange. The later is better known under the name of fire opal.  Many further sub-classification are done based on the quality and the distribution of the color patches.  The most famous opals varieties are: harlequin, pinpoint, flash, peacock, and flame.  Some types of opals do not display this play of color. Among those there is the bulk of orange fire, a green variety resembling chrysoprase (called prase opal), and a blue green variety found in Peru.

Due to the sedimentary process resulting from the concentration of silica gel in evaporating seas, opal always contains an amount of water ranging from 3% to 10% of the total weight of the stone. This is the reason why it should be protected from heat or strong light, which can dry the water out, causing cracks. Also ultrasonic cleaners, acids, or any strong solvents should be avoided.

Opal is the birthstone for October alternating with tourmaline.

Sources

Ancient opals came from mines in what is now Eastern Slovakia, but these deposits are now exhausted.

Since gold prospectors in Australia found the first pieces of precious opal in 1863, production has now increased to reach 98% of the world market. Queen Victoria was one of the first to appreciate opals from this new source.

Black opal is found in the district Lightning Ridge, New South Wales.  Another large opal producing area is Coober Pedy, South Australia, which produces white opal. The name Coober Pedy is an Aboriginal name meaning “white man in a hole,” which accurately describes the living condition of the miners.  Andamooka is known for producing mostly white material.  Coober Pedy and Andamooka together produce more than 90% of the gem quality opal.

Fire opal is produced in Mexico.