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Gemstones

Articles from GEMSTONES - TOPAZ (5 Articles)











Swiss blue Nigerian topaz (left) | Swiss blue topaz (right)
Swiss blue Nigerian topaz (left) | Swiss blue topaz (right)

Colour investigation:Topaz

Topaz has been known as a gemstone for at least 2,000 years. It occurs in a range of colours and has hardness of 8 on the Mohs scale, however it can be brittle. JENNIFER SUGGETT investigates.

Imperial topaz is yellow, pink or pink-orange and is the most valuable of the topaz colours. It comes from a single mine in Brazil discovered in the late 1700s and was named after the then Portuguese royalty (hence Imperial). Topaz is pleochroic and thus often shows three colours with two distinct, and one of weaker hue.

Large deposits are found in many countries including, Brazil, Sri Lanka, Russia, Burma, Nigeria, Australia and the US, making it an inexpensive gem. Colourless, blues and champagnes are found in NSW and Queensland. It is also found in flawless and large crystals enabling it to be faceted into gems weighing thousands of carats. The blues and yellows have been used to imitate both aquamarine and citrine, while white topaz has long been used as a simulant for diamonds.

Topaz has a chemical formulae of Al2(F,OH)2SiO4. Impurities of trace elements and structural defects can contribute to the colour in topaz; chromium as a replacement for aluminium is the colouring element for red and pink topaz.

"Large deposits are found in many countries including, Brazil, Sri Lanka, Russia, Burma, Nigeria, Australia and the US"

As with many popular gems, topaz is treated to improve its colour; in its natural state topaz is yellow to brown. Naturally occurring blue topaz is rare thus colourless, grey, pale yellow and pale blue stones are heat treated, irradiated or both to produce a darker blue.

Brazilian reddish brown gems become colourless when slowly heated to 400 degrees and then on slow cooling they develop a pink to purple red colour. If heated to 600 degrees they become colourless.

Topaz is irradiated by gamma rays followed by heating to about 200 degrees, the radiation produces a greenish-brown colour. Some stones will turn a deeper blue than found naturally and others become colourless. The gemstone can be treated via a vapour deposition process to give it a rainbow effect on the stone’s surface and is called “Mystic topaz”.

The only official variety of topaz is Imperial topaz which is the most valuable. However, there are a number of other names used in the jewellery trade:

  • Azotic topaz – orange-pink with a rainbow like colour effect from vapour deposition.
  • Mystic topaz – multi-coloured with a rainbow like effect from vapour deposition.
  • Swiss blue topaz - sky blue colour, but lighter than London Blue.
  • London blue topaz – deep blue colour.
  • Rutilated topaz – similar to rutilated quartz but the inclusions of “rutile” in topaz are actually limonite, not rutile.
  • Sherry topaz– light orange brown to brownish-pink in colour.
  • Silver topaz and white topaz – colourless top

Topaz and citrine are the birthstones of November. Yellow topaz is often confused with citrine due to its similar colour. Since topaz comes in many colours, the birthday choice colours are numerous. The variety of colours, good hardness, transparency, and abundance make topaz a popular choice for jewellery.











ABOUT THE AUTHOR
Jennifer Suggett

Jennifer Suggett FGAA is a qualified gemmologist and gemmology lecturer. She loves to introduce people to the wonderful world of gemstones particularly the lesser-known gemstones. For information on gemstones, visit: gem.org.au









Thursday, 18 October, 2018 12:38pm
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