Opal is primarily composed of non-crystalline silica (SiO2) with five to ten percent water. It is formed near the Earth's surface in volcanic rock, as water dissolves silica and percolates into cavities and cracks in the hot rock, then evaporates.
As a result of this non-rigid, non-crystalline structure, opal is brittle and breaks and scratches easily. It has a mineral hardness of only 5 1/2 to 6 1/2. However, its unusual beauty makes it extremely popular as a gemstone. Microscopic silicon and oxygen spheres in the stone, which are roughly equal in size and distribution in gem-quality opal, has the effect of diffracting light at various wavelengths and angles, creating a rainbow of colors as the stone is moved under light. Certain colors may also be produced by iron (yellow and red) or magnesium (green and blue) oxides within the stone. This phenomenon is called "play of color" or "opalescence" and varies greatly from stone to stone; larger splotches of color result in a more desirable and expensive gem.
Black opal is most popular and treasured, but opal comes in dozens of colors and varieties. White opal has a light background and pastel colors; crystal opal has a colorless background; fire opal has a translucent yellow, orange, or red background color. Jelly or water opal is colorless and transparent but has very little play of color. Solid opals (cabochons) are most valuable, but the desired color effects can also be produced by combining a thin strip of opal with a colored backing (doublet) to reduce its transparency or additionally with a clear quartz layer (triplet). The quartz layer protects the brittle opal and is ideal for rings and other "high contact" jewelry.
Certain opals may unpredictably develop internal cracks when exposed to a sudden change in light, heat, and/or humidity, or when it is subjected to vibration during cutting and polishing of the gem. This condition is called "crazing" and much care must be taken to keep stones stable while preparing a gem.
Opal's name originates in the Greek word opallios, meaning "to see a change of color" and a modification of the Sanskrit name for the gem, upula. The Roman historian Pliny described opal as "made up of the glories of the most precious stones. To describe it is a matter of inexpressive difficulty: There is in it the gentler fire of the ruby, the brilliant purple of the amethyst, the sea-green of the emerald, all shining together in an incredible union."
Opal was treasured in the England of Queen Elizabeth I and was described by Shakespeare in Twelfth Night as the "queen of gems." However, the 1831 novel Anne of Geierstein by Sir Walter Scott described it as a stone of evil, which shone red when Anne was angry, blue when sad, and green when happy, finally fading to ashen grey when she died. Opal had difficulty shaking this reputation until Queen Victoria made a present of opal jewelry to her children, making the stone popular again.
The Romans considered opal a symbol of hope and purity, while the Greeks thought it bestowed the power of foresight and prophecy. The Arabs believed that opals made their wearer invisible and that they were fallen from heaven in flashes of lightning, making it sacred. The country of India had a not-dissimilar legend, that the gods Brahma, Vishnu, and Shiva once fell in jealous love for the same woman and bestowed upon her the colors blue, gold, and red to distinguish her as their own. She died nevertheless, and the Eternal took pity on her and transformed her into an opal stone with all three colors.
In medieval Europe, opal was believed to maintain a strong heart, prevent fainting and infection, and cleanse the air. However, after the Black Death began to sweep across the continent, the gem was sometimes believed to be the cause of death due to its change in appearance and luster. (In actuality, any color change was probably a symptom of death, due to the high heat sensitivity of the stone.) Even today it is sometimes considered bad luck to wear opals, at least if it isn't your birthstone.
The aborigines of Australia believe the opal is a half-serpent, half-human devil lurking in the ground and luring mean to their deaths with colorful flashes of magic. However, the miners in that country have a legend that a huge opal controls the stars and human love, as well as the gold in the ground.
The principal source of black opals today is Australia, while fire opals are mined and sold primarily in Mexico. Opal is also mined in Nevada and the nearby United States, Brazil, Honduras, Guatemala, Nicaragua, Japan, and Ireland. Tanzania and Ethiopia recently discovered sources of opal as well.
Synthetic opal has been developed throughout the twentieth century and laboratory opals are now of very high-quality. These synthetic stones are made of exactly the same material as natural stones and under very similar conditions, first arriving on the open market in 1974. A skilled gemologist must inspect a stone under a microscope to distinguish a synthetic opal from a natural one.
Opal is the birthstone for the month of October (together with tourmaline) and is the symbolic gemstone for the 14th and 34th wedding anniversary.