Kansas State Gemstone
Opal is not a true mineral in the accepted sense of the word because it does not have a crystal structure. Considered a mineraloid gel, opal is a hydrated silicon dioxide. The stone’s base color can be white, yellow, orange, red, purple, blue, green, gray, brown, black, or colorless. White and shades of green are the most common, while red against a black background is the rarest. Certain specimens of opal display exceptional variations in color or iridescent flashes of color when turned or viewed from different directions. This phenomenon is called "play of color."
Opal has over one hundred variety and trade names, but the list of accepted or commonly used names is much shorter. The most important and most widely known opal is the precious opal, defined by its play of color phenomenon, which makes it valuable. Fine specimens with intense play of color can be worth more than diamond. The density and pattern of the silica spheres that compose precious opals are arranged in such a way that when light is diffracted through the stone, it causes a color sheen. Precious opal may be subdivided further by color modifiers that describe the body color, such as white, black, pink, and blue. Opals not exhibiting play of color are called common opals and are much less valuable. This milky appearance of a common opal is called opalescence.
Moss opal is a variety containing manganese oxide impurities that form dark, branching deposits. The branches give the stone a mossy appearance. Unlike other varieties of opal, moss opal is found in only a handful of locations around the world.
Opal is brittle, heat sensitive, and breaks and scratches easily. It can develop internal and external cracks if it is dried too quickly or exposed to heat over a long period of time. The stone is three to 30 percent water, and some varieties self-destruct through the loss of water. Despite these drawbacks, opal’s unsurpassed beauty guarantees its status as a premier gemstone.
Although Kansas does not have an official state gemstone, moss opal represents the state well as it is found in the state’s chalk beds.
Reportedly, opal’s name evolved from the Roman word opalus from the Greek word οπάλιο (opalio), meaning "to see a change of color." The Greek word was a modification of the ancient Indian Sanskrit name for opal, upala, which meant "precious stone."
FORMATION AND OCCURRENCE
Opal is deposited at relatively low temperatures and may occur in the fissures of almost any kind of rock. It is most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
Opal has centuries of history as a treasured gemstone. Historically, beliefs associated with the wearing of opal have varied. Early Greeks thought that opals gave their owners the powers of foresight and prophecy, and Romans adored it as a token of hope and purity. Eastern people regarded it as sacred, and Arabs believed it fell from heaven. Australian Aboriginal legend says that opals were born at the spot where the creator came to earth on a rainbow in order to bring peace to humans.
In the Middle Ages, opal was thought to possess all the characteristics of all the gemstones represented by the many colors of opals, and therefore to provide great luck. In the 19th century, superstitions grew about the bad luck or fate that could befall one for wearing opal if it were not the wearer’s birthstone. Today, these superstitions have diminished, but some people still believe it is bad luck to wear opals. Opal reached a height of popularity in the Art Deco period.
Precious opal is the primary gem form of this stone, but gems are cut from common opal as well. Opal is most commonly cut into cabochons and used in all types of jewelry. The most desired variety of opal is the black opal, which has a dark green, dark blue, or black background. Also important is white opal with its white-, yellow-, or cream-colored background. Mexican fire opal is yellow-orange to red, transparent to translucent, and opalescent. When Mexican fire opal displays play of color, it is called precious fire opal. Fire opal is the second most important opal commercially.
Opal is thought to have healing powers, including relieving depression and improving eyesight. It is also said to help one find inner beauty and true love.
Opal is considered the birthstone for people born in October or under the zodiac signs of Libra and Aquarius. It is the mystical birthstone for April and the gemstone for the 14th and 18th years of marriage.
SUBSTITUTES / SYNTHETICS
Following the discovery of the ordered sphere structure of precious opal, factories and laboratories began producing synthetic opal. Imitation opal can be distinguished from genuine opal as it does not fluoresce under UV light and is more porous and because its patches of color are quite regular. Some imitation opals are made of glass or plastic.
Crystal Structure: Irregular veins, in masses, in nodules
Hardness (Mohs): 5.5–6.5
Color: White, black, red, orange, most of the full spectrum, colorless, iridescent
Transparency: Transparent, opaque
Luster: Vitreous, waxy, greasy, dull
Refractive Index: 1.450 (+.020, -.080) Mexican opal may read as low as 1.37, but typically reads 1.42–1.43
Density: 2.15 (+.08, -.90)
Fracture: Conchoidal to uneven
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press