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 also 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. Many of the earliest gem traders did not realize that these yellowish stones were actually different materials.
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.
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 can be 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 today 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, but these colors can also be 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 can wear thin or wear 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.
The NRC has no reason to believe that wearing irradiated gemstones can be harmful. There have been no reported cases of anyone being harmed by wearing them. There is no safety reason to stop wearing blue topaz or any other irradiated gems.
Do I need a license to sell blue topaz or other irradiated gems?
Probably not. NRC regulations cover material made radioactive in a nuclear reactor or linear accelerator. The initial transfer of these materials must be done under an NRC distribution license. Once the radioactivity levels fall below certain limits, the materials become "exempt" from further regulation. This means no license is needed to buy or to resell them. So individual jewelers do not need to be licensed as long as they receive their stones from an NRC licensee.
Radioactive Blue Topaz?
The type of irradiation used to transform colorless topaz into material with a blue color can cause some irradiated topaz to become slightly radioactive. Fortunately, the radioactivity level of the topaz begins to decline as soon as treatment is complete. It eventually declines to a level that is safe for the topaz to be handled during manufacturing and be sold to the public in jewelry.
In the United States, the Nuclear Regulatory Commission requires all irradiated gems and gem materials to be securely stored until their radioactivity decays to a level that is safe for manufacturing and sale. This is done to protect employees of the gem and jewelry industry and the jewelry-buying public.
All companies who distribute newly irradiated gems in the United States must be licensed by the Nuclear Regulatory Commission. They must also conduct radiological surveys of all materials in secure storage to be sure that no gems are released until their radioactivity declines to a level that will not pose any health risks.
The Nuclear Regulatory Commission has detailed information about irradiated topaz and other gemstones on their website. They also have answers to frequently asked questions. Two answers that we believe will be of interest to our readers are quoted in the box on this page. You can read the rest by visiting the NRC website.
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.
Much topaz grows as crystals within the veins and voids of igneous rocks. This topaz is then found in the cavities of a pegmatite, or in the vesicles and intergranular spaces of rhyolite. These topaz crystals grow during the late stages of magma cooling and while degassing releases the fluorine necessary for topaz crystal growth.
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.
Author: Hobart M. King, Ph.D.
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