Topaz, a semi-precious gemstone used in jewellery, is available in an array of colours including yellow, pink, lavender, reddish-orange, red and brown. The most popular colour today is blue, which is inexpensive compared to aquamarine and sapphire because it is produced from plentiful supplies of pale blue or colourless topaz through heat and irradiation. Blue topaz serves as December's birthstone and, since 1969, as the state gem of Texas.
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Topaz Scientific Data
Topaz, a mineral composed of aluminium fluorosilicate, boasts a hardness of 8 on the Moh's hardness scale, regardless of colour. This scale measures scratch resistance with 10 being diamond and 1 being talc. Blue and colourless topaz share a specific gravity (measuring density compared to water) of 3.56 to 3.57 and may exhibit a yellow or greenish glow when placed under long-wavelength ultraviolet light. Topaz's perfect-one-plane cleavage requires rotating the stone 5 to 10 per cent off the cleavage plane prior to cutting. The gem remains stable as long as it is not cleaned with steam or ultrasonics.
Where to Find Blue Topaz
Naturally blue topaz occurs in pegmatites, which are coarse-grained, mineral-rich volcanic rock seams or pods that formed later or more slowly than their surroundings. Mines in Brazil, Siberia, Zimbabwe and Burma produce blue topaz. In the United States, blue topaz has been discovered and mined near Pike's Peak, Colorado and in Mason County in central Texas. Jewellers celebrated the Texas find by creating a special Lone Star cut. The natural blue topaz is pale and aquamarine-like --- without the green tint of that gem's colour.
Making Topaz Blue(r)
Naturally pale blue or colourless topaz undergo one or more processes to create the rich, medium-blue topaz currently in demand. A light but rich blue known to gemologists as "Cobalt Blue" results from exposure to a gamma ray source that produces blue and yellow colour centres, followed with heating to remove the yellow. Linear accelerator exposure to a high energy electron beam followed by water cooling and additional heating produces a deeper blue with no grey called "Sky Blue." These stones remain radioactive for several weeks after treatment and are stored until safe. The third process exposes topaz to a nuclear reactor, producing the medium to dark greyish-blue called "London "Blue" and requires additional heating only to remove the grey cast. These stones remain radioactive for months. Combining linear accelerator and nuclear reactor exposure and heating produces the "Swiss Blue" colour, a rich dark blue with no grey. Any colour can result from any process, so no colouring records are kept for individual stones.
No definitive proof exists for where the name originated, being variously described as rooted in Sanskrit or Greek, or related to the ancient name for an island in the Red Sea where peridots were mined. Egyptians and Roman connected yellow topaz with the sun. Folklore ascribes to topaz the ability to increase intelligence and creativity, protect one's health and aid the development of one's spirituality, so protect your blue topaz from chipping and sudden extreme temperature changes and perhaps it will protect you.
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- University of Texas at Austin: Dept of Geological Sciences; Topaz; August 20, 2009
- Gem Trade Net: Education: Blue Topaz
- University of Texas at Austin: Dept of Geological Sciences: Mineral Overview
- Gem Society: Precious and Blue Topaz
- Project Alert NASA/CSU: Specific Gravity
- The Pegmatite Page: Home Page