Precious opals are known for their opalescence or fire—the streaks of iridescent color that change according to the angle of observation. This opalescence is caused by the opal’s internal structure. Using electron microscopes, it was discovered that opals consist of tightly packed transparent spheres of silica (each one less than 1/1000 mm), with the spaces between filled by air or water. In precious opal, these spheres are all the same size and stacked in layers in a very orderly three-dimensional grid that breaks visible white light into separate colors. (Larger silica spheres are associated with red; smaller sphere with green to purple.) This opal forms when a rock cavity holds a clean solution of silica from which, over thousands of years, water is slowly removed. As the water evaporates, the silica spheres form and slowly settle, arranging themselves in the grid, which is later “glued” together by additional deposits of silica. In the common, non-gem opal, the silica spheres are not uniform in size and/or their arrangement is disorderly.
Volcanic opals form when silica-bearing solutions deposit silica in holes that were formed when gas bubbles became trapped in the cooling lava. These opals generally don’t have as much fire as the opals that form in sedimentary rock.
There are two main groups of precious opals: white or milky opals, which have a white or light body tone (or base color); and black opals, which have a darker body tone—dark grey, dark blue, dark green, or grey-black. A true black base color is exceptionally rare. Until the early 1900s, the main source for opals was Czechoslovakian volcanic deposits. Currently, Australia produces over 90% of world’s precious opals, with black opals coming from the Lightning Ridge opal fields.
Fire opal, also known as sun opal, is not opalescent. Most fire opals are milky, but the gem-quality fire opals are clear, transparent reddish-orange stones. These are found in Mexico, Guatemala, Brazil, Honduras, the United States and Turkey, and are, in general, the only opals cut with facets.
In the Orient opals were considered stones of hope and loyalty. In the west, their lore has been more ambiguous. Opal has been known as the thief’s stone, at least since the time of Plato. In his Republic, Plato tells the story of Gyges, a poor shepherd, who found an opal ring that gave him the power of invisibility. With the help of the opal ring, Gyges stole the queen and the crown of the king of Lydia. Later magical traditions claimed that opal didn’t exactly make the wearer invisible; rather it obscured the sight of anyone who gazed on the wearer of the stone, and it simultaneously sharpened the wearer’s sight, thus making a thief’s work much easier.
The beliefs in opal’s effect on the eyes also extended to healing. According to Ben Johnson (1572-1637), opal was believed to both cure eyes diseases and strengthen vision, especially if wrapped in a bay leaf. Opals were also said to turn pale in the presence of poison, ease melancholy, and protect the wearer from contagious diseases.
Pliny believed that the opal was the “vessel of unity” from which other gems took their color – carbuncle (ruby/garnet) drank its embers; amethysts took their deep purple, emeralds their green, topazes their yellow, and sapphires their blue. The Romans considered opal a gem of good fortune and an aid in prophecy; only the emerald was considered more valuable.
There are, of course, quite a number of superstitions about opals being unlucky. Some of these may stem from the ancient belief that opals exist in a kind of sympathy with their owner and will reflect the owner’s anger (in their fire) or illness (by growing dull or cracking). Another belief of this sort arose in the 14th century during the Black Death when it was said that an opal would flare brilliantly before its owner’s death and then grow dull once the owner died. Sir Walter Scott’s novel Anne of Geierstein (1829) played on these beliefs, making them even more dire in his tale of the princess Hermione, who wore an enchanted opal in her hair. In the novel, holy water falls on the opal and Hermione falls into a faint. The next day her husband finds only ashes on her bed and in those ashes, the wicked stone. A more mundane explanation for the opal’s bad reputation is that jewelers—who were held responsible for stones breaking-- spread the tale of it being unlucky so they wouldn’t have to cut or set such a fragile gem. Despite all these dark beliefs, Queen Victoria loved opals, gave them as wedding presents, and managed to restore them to favor.
Today, opal is considered a stone of good fortune, as well as an aid in astral projection and connecting to the wearer’s psychic powers. Metaph

Color: White, grey, blue, green, orange
Chemical composition: SiO2.nH2O hydrous silicon dioxide
Mohs’ hardness: 5 ½-6 ½
Transparency: Transparent, opaque
Luster: Vitreous to pearl
Fracture: Conchoidal, splintery, brittle
Crystal system: Amorphous, kidney or grape-shaped aggregates
Cleavage: None
Specific gravity: 1.98-2.20
Refractive index: 1.44-1.46
Dispersion: None
Pleochroism: None
Fluorescence: White: white, bluish, brownish, greenish
Black: usually none
Fire: greenish to brown
Occurrence: Australia, Brazil, Czechoslovakia, Mexico, Peru, United States

The name opal originated with the Sanskrit upala, which means precious stone. The Greek derivative opallios means “to see a change in color. The Romans called it cupid paederos, “child beautiful as love,” and ancient Arabic writings explained that opals contained bits of lightning that the stones absorbed when lightning struck the ground. The Mayas and Aztecs called it “bird of paradise stone” and “hummingbird stone.”
Opal is hardened silica gel—or a hydrated form of silica. It occurs in veins in volcanic rocks or it fills cavities in arid, sedimentary areas, and is most commonly found in sandstone, rhyolite, limonite, and basalt. It also forms stalactites and stalagmites and can replace organic matter in fossilized wood, bone and shell. Because its internal structure is not crystalline, opal is considered a mineraloid, rather than a mineral.
Mineralogists divide opals into three groups: the precious opals, which show opalescence--flashes of color (iridescence); the yellow-red fire opals; and the common (or “potch”) opals. All opals contain water, though the amount of water varies. Visually, the three groups are dramatically different from each other, as they have quite different physical properties

The darker opals look spectacular with nearly all the green and blue stones—including green tourmaline, emerald, green aventurine, peridot, aquamarine, blue topaz, azurite, sapphire, and tanzanite--as well as amethyst and sugilite.