- Related Topics:
- fire opal
- hydrophane
- black opal
- siliceous sinter
- white opal
opal
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What is opal?
Opal is a silica mineral used as a gemstone. It is a submicrocrystalline variety of cristobalite. It is a hydrous silica that is softer and less dense than quartz.
Typically colorless, disseminated impurities give opal a range of dull body colors including yellows, reds, and black. Precious opals are translucent to transparent, with a milky or pearly opalescence and an attractive play of many colors caused by light interference along minute cracks and internal inhomogeneities.
Opal forms from silica dissolved in water over long periods. It is deposited from circulating waters as nodules, veinlets, and encrustations in nearly all kinds of rocks.
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What does opal look like?
Opal, a silica mineral, is a gemstone composed of submicrocrystalline cristobalite. While fundamentally colorless, impurities create body colors ranging from yellows and reds (from iron oxides) to black (from manganese oxides and organic carbon).
Precious opals are translucent to transparent, displaying milky opalescence and a play of many colors that change as the stone is viewed from different angles. This effect results from light interference along internal cracks. Black opal is especially rare and prized, while white opal and fire opal (yellow, orange, or red) are more common.
Opal forms from silica-bearing solutions and is found in varied forms within many rock types.
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How is opal different from other minerals?
Opal stands out from other minerals because it is a poorly crystalline or amorphous hydrous silica mineral, meaning it lacks a defined crystal structure. Unlike many crystalline minerals, opal is composed of spheres of hydrous silica arranged in hexagonal or cubic patterns. The size differences of these spheres cause light refraction, resulting in a mysterious, almost holographic effect.
Additionally, opal contains 4 to 9 percent water, a variable amount compared to other silica minerals. It forms through precipitation from silica-bearing solutions near the Earth’s surface and can replace other substances like wood and bone, creating opal fossils. This mineral is known for its play of color, opalescence, and varied body colors, making it a prized gemstone.
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Where is opal commonly found in the world?
Opal, a silica mineral used as a gemstone, is found worldwide in various kinds of rocks but is most abundant in volcanic rocks and hot-spring areas.
The finest gem opals come from Australia, specifically South Australia, Queensland, and New South Wales. Lightning Ridge is known for black stones. White opal can be found in Japan, and fire opal in Mexico and Honduras. The western United States, India, and New Zealand also have precious opal deposits.
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How does opal form in nature?
Opals form over centuries through the deposition of silica dissolved in water. This process occurs near the Earth’s surface, typically in volcanic rocks and areas with hot spring activity. The dissolved silica precipitates from silica-bearing solutions.
Many opals consist of tiny hydrous silica spheres arranged in hexagonal or cubic patterns. Light diffraction from these regularly oriented spheres causes the play of color seen in precious opals. Impurities can give opals various body colors, while gas-filled cavities cause the milkiness of white and gray opals.
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What are some popular uses for opal?
Opal is a silica mineral used as a gemstone. While fundamentally colorless, impurities create body colors ranging from yellows and reds (from iron oxides) to black (from manganese oxides and organic carbon).
Precious opals, admired for their pearly opalescence and play of colors, are used in jewelry. Fire opals are usually facet cut, but most other precious opals are finished en cabochon because their optical properties are best displayed on smoothly rounded surfaces.
Common opal is widely mined for abrasives, insulation, fillers, and ceramic ingredients. Undersized fragments are used for inlay work, and small pieces scattered throughout a natural matrix are commonly sold under the name root of opal.
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opal, silica mineral extensively used as a gemstone, a submicrocrystalline variety of cristobalite. In ancient times opal was included among the noble gems and was ranked second only to emerald by the Romans. In the Middle Ages it was supposed to be lucky, but in modern times it has been regarded as unlucky.
Opal is fundamentally colourless, but such material is rarely found. Disseminated impurities generally impart to opal various dull body colours that range from the yellows and reds derived from iron oxides to black from manganese oxides and organic carbon. The milkiness of many white and gray opals is attributable to an abundance of tiny gas-filled cavities in them. Black opal, with a very dark gray or blue to black body colour, is particularly rare and highly prized. White opal, with light body colours, and fire opal, characterized by yellow, orange, or red body colour, are much more common.
Precious opals are translucent to transparent and are distinguished by a combination of milky to pearly opalescence and an attractive play of many colours. These colours flash and change as a stone is viewed from different directions and are caused by interference of light along minute cracks and other internal inhomogeneities.
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Opal is deposited from circulating waters in such varied forms as nodules, stalactitic masses, veinlets, and encrustations and is widely distributed in nearly all kinds of rocks. It is most abundant in volcanic rocks, especially in areas of hot-spring activity. It also forms pseudomorphs after wood and other fossil organic matter and after gypsum, calcite, feldspars, and many other minerals that it has replaced. As the siliceous material secreted by organisms such as diatoms and radiolarians, opal constitutes important parts of many sedimentary accumulations.
The finest gem opals have been obtained from South Australia, Queensland, and New South Wales in Australia; the Lightning Ridge field is famous for superb black stones. Deposits of white opal in Japan, fire opal in Mexico and Honduras, and several varieties of precious opal in India, New Zealand, and the western United States also have yielded much gem material. Most of the precious opal marketed in ancient times was obtained from occurrences in what is now Slovakia. Various forms of common opal are widely mined for use as abrasives, insulation media, fillers, and ceramic ingredients.
Fire opals usually are facet cut, but most other precious opals are finished en cabochon because their optical properties are best displayed on smoothly rounded surfaces. Undersized fragments are used for inlay work, and small pieces scattered throughout a natural matrix are commonly sold under the name root of opal. Because opal may crack or lose its colour if it dries, many finished stones are protected by water or films of oil until they are sold. Opals absorb liquids very readily. An extremely porous variety, known as hydrophane, can absorb surprising quantities of water; it is almost opaque when dry but nearly transparent when saturated. Light-coloured stones are often dyed to resemble rarer, more deeply coloured varieties.