A gemstone that occurs in a wide range of natural and treated colors.
What is Topaz?
Topaz is a rare silicate mineral with a chemical composition of Al2SiO4(F,OH)2. It usually forms in fractures and cavities of igneous rocks such as pegmatite and rhyolite, late in their cooling history. It is also found as water-worn pebbles in stream sediments derived from those igneous rocks.
Topaz is a well-known gemstone sold in a wide variety of attractive colors. Some of these colors are natural, while others are produced by treating pale or colorless topaz with heat, radiation, or metallic coatings.
Physical Properties of Topaz
|Color||Natural colors include: colorless, yellow, orange, red, pink, blue, green. Occurs in a wide range of treated colors, most often blue.|
|Streak||Colorless - harder than the streak plate.|
|Diaphaneity||Translucent to transparent.|
|Cleavage||Perfect basal cleavage.|
|Specific Gravity||3.4 to 3.6|
|Diagnostic Properties||Hardness, prismatic crystals, sometimes striated, cleavage, specific gravity.|
|Uses||Gemstone, Mohs hardness index mineral.|
Physical Properties of Topaz
One of the best-known physical properties of topaz is its hardness. It has a hardness of 8 on the Mohs hardness scale. It also serves as the Mohs hardness scale index mineral for a hardness of 8. Every student who takes an introductory geology course learns about the hardness of topaz. Diamond, corundum, and chrysoberyl are the only commonly-known minerals that are harder.
Most topaz is colorless or milky. Yellowish and brownish colors are also common. Natural pink, orange, red, purple, and blue topaz are rare and valuable if they are of gem quality.
When allowed to grow unrestricted, topaz forms orthorhombic crystals, often with striations that parallel the long axis of the crystal. It also has a distinct basal cleavage that breaks perpendicular to the long axis of the crystal. This cleavage makes topaz a more fragile gemstone than its hardness of 8 would imply. Hardness is the resistance to being scratched, but the ability to resist breakage is a property known as tenacity.
Topaz has a specific gravity that ranges between 3.4 and 3.6. This is quite high for a mineral composed of aluminum, silicon, and gaseous elements.
Use of Topaz as a Gemstone
The name "topaz" and many language variants have been used for yellowish gemstones for at least two thousand years. At that time all yellowish gems were called topaz in many parts of the world. People who traded in gems did not realize that these yellowish stones were of many different kinds.
Then, about two hundred years ago, people who traded in gems began to realize that these yellowish gems might be topaz, quartz, beryl, olivine, sapphire, or one of many other minerals. They also learned that topaz occurred in a wide range of colors other than yellow. Today, gemologists know the full range of colors for natural topaz.
If you visited a jewelry store fifty years ago and asked to see topaz, you would likely be shown gems that were in the color range of yellow, orange, and brown. Starting in the 1970s and 1980s, the most common color that you would be shown began to be blue. This blue color was usually produced by treatments that converted colorless topaz into a more marketable gemstone.
Today most topaz offered in mall and department store jewelry stores at low to moderate price levels has been treated in a laboratory. Colorless topaz is heated, irradiated, and coated with thin layers of metallic oxides to alter its color.
Natural blue topaz is extremely rare and is usually pale blue. Almost all of the blue topaz offered in stores is colorless topaz that has been irradiated and then heated to produce a blue color. "Swiss blue" and "London blue" are trade names for two of the most common varieties of treated blue topaz seen in today's market.
Natural pink to purple topaz is also extremely rare. These colors are often produced in a laboratory. The starting point is a stone cut from colorless topaz. It is first heated and then coated with a layer of metallic oxide to produce the pink color. If coated stones are worn in jewelry, over time the coating often wears thin or wears through at points on the stone where abrasion occurs.
Some topaz is coated with a metallic oxide that gives the stone a multicolored iridescent luster. These stones, known as "mystic topaz," appear to change color if the observer moves the stone under a light or changes the angle of observation. These coatings are also thin and can be worn through during wear.
Geologic Occurrence of Topaz
Topaz has a chemical composition of Al2SiO4(F,OH)2. The fluorine in its composition is a limiting factor on its formation. Fluorine gas in concentrations high enough to form minerals only occurs in a few geologic environments.
Most topaz forms the veins and cavities of igneous rocks such as pegmatite and rhyolite. It forms during the late stages of magma cooling while hydrothermal activity delivers fluorine. Topaz occurs in pockets within pegmatites, in fractures that carried hydrothermal fluids, and in gas cavities within rhyolite.
Precipitating in cavities, topaz sometimes develops nicely-formed crystals. These crystals can have excellent clarity and can be used as a gem material. Especially attractive crystals of topaz are popular with mineral collectors. They have the value of a mineral specimen and the value of a gem material.
Topaz is also found as water-worn pebbles in stream sediments derived from the weathering of pegmatites and rhyolites. These are often produced by placer mining.
Topaz is found in many locations worldwide where rocks like pegmatite and rhyolite are formed. It is only a minor mineral at these locations, and it is considered to be a rare mineral on the basis of its general abundance.
Brazil is the leading source of gem-quality topaz today. Sri Lanka is another important producer. Topaz is also produced in Nigeria, Australia, Pakistan, Russia, India, Zimbabwe, Madagascar, and Namibia. In the United States, Utah named topaz as its state gemstone.
Contributor: Hobart King
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