The Braganza, the largest stone in the Portuguese Crown Jewels, weighs 1,680 carats and was believed for centuries to be a diamond. It was set in the Portuguese crown alongside rubies and diamonds as an equal. Modern gemological examination established that it is colourless topaz. The stone was found in Brazil in the late 18th century and brought to Portugal, where its size and apparent brilliance convinced everyone who examined it that it must be diamond. This misidentification was not merely a failure of gemological knowledge: topaz, like diamond, crystallises in a form with high lustre and clarity, reaches large sizes without inclusions, and when colourless has a visual character that, before the refractometer, was difficult to distinguish from diamond for a large stone. The Braganza is now understood to be one of the finest examples of imperial-quality colourless topaz, its misidentification for two centuries a testament to topaz's optical quality. It remains in the Ajuda National Palace in Lisbon.
Quick answer: what is topaz? Topaz is an aluminium fluorosilicate mineral with the formula Al2SiO4(F,OH)2, crystallising in the orthorhombic system. It has Mohs hardness 8 and perfect basal cleavage, meaning it can split cleanly along a plane perpendicular to the c-axis with a sharp blow. Topaz occurs in many colours: colourless (the most common natural state), orange-yellow to brownish-orange (imperial or precious topaz), pink, blue (natural, rare), sherry brown, and green. Commercial blue topaz is almost always irradiation-treated colourless topaz. Sources: GIA Gem Reference Guide (2006), pp. 96-99; Wise, R.W., Secrets of the Gem Trade (2016), pp. 306-318.

Chemistry, hardness, and the cleavage problem

Topaz's Al2SiO4(F,OH)2 formula makes it an orthosilicate with aluminium in octahedral coordination and fluorine and hydroxyl occupying additional sites. The fluorine content is partially responsible for topaz's perfect basal cleavage, one of the most significant practical concerns for this gem species (GIA Gem Reference Guide, 2006, pp. 96-97; Klein, C., Manual of Mineral Science, 2002).

Perfect basal cleavage means topaz has a planar weakness perpendicular to the c-axis that will split cleanly with a sufficiently sharp blow. The cleavage plane is parallel to the table facet in most topaz cuts, meaning the stone's face-up surface is the same plane as the cleavage direction. A ring stone of topaz struck on the table can cleave cleanly in half. This is not a rare failure mode: lapidaries who cut topaz routinely lose stones to accidental cleavage during cutting, and finished topaz rings need protective settings. The combination of Mohs 8 (excellent scratch resistance) and perfect cleavage (impact vulnerability) makes topaz a different wear proposition from corundum, which has the same hardness but no significant cleavage (GIA; Wise, 2016).

Topaz varieties: commercial context and treatment status Variety Natural colour character Treatment status Commercial tier Imperial topaz Orange, yellow-orange, sherry Natural colour; no standard treatment Premium fine gem Pink topaz Pink to pinkish-orange Natural; some heat treated from brown Good commercial Blue topaz Blue (natural is pale; vivid is treated) Irradiation + heat; universally treated Commercial commodity Colourless topaz Colourless; most common natural state None required Low commercial value Green / mystic topaz Coating-produced rainbow effect Thin-film coating; not stable Fashion jewellery only Source: GIA; AGTA treatment codes; Wise (2016). Imperial topaz and natural pink are the fine gem tiers; blue is treated commodity.

Topaz varieties and treatment status. Imperial topaz (orange, natural colour) and pink topaz are the fine gem tiers. Blue topaz is universally irradiation-treated colourless topaz and is a commercial commodity. Mystic topaz has a thin-film coating that is not durable. Source: GIA; AGTA; Wise (2016).

Imperial topaz: the fine gem variety

Imperial topaz is the trade designation for topaz in the orange to sherry-brown to pinkish-orange colour range, produced naturally by colour centres involving iron in the crystal structure. The primary and most celebrated source is Ouro Preto (Portuguese for "black gold," named for the gold mining that brought Portuguese colonisation) in Minas Gerais, Brazil. The Ouro Preto mines have produced imperial topaz for centuries; they remain the primary source for the finest colour material (GIA; Wise, 2016, pp. 306-310).

Imperial topaz at its finest shows a vivid, saturated orange with a pinkish secondary hue, or a pure orange-gold, or a deep sherry (brownish orange-red). The colour at the finest quality is warm and rich without being brownish or dull. Eye-clean imperial topaz of fine colour above 5 carats is genuinely uncommon and commands per-carat prices of USD 500-3,000 depending on colour and size. The name "imperial" has no official definition; it is a trade term with no standard threshold, applied variously by dealers. The key quality factors are colour saturation, colour character (orange vs brownish vs pink), and clarity (GIA; Wise, 2016).

Blue topaz: the treatment that changed a market

Natural blue topaz exists but is pale and commercially uncommon. The vivid blue topaz that saturates the commercial jewellery market, in shades from pale sky blue ("Sky Blue") through medium blue ("Swiss Blue") to deep vivid blue ("London Blue"), is almost entirely irradiation-treated colourless topaz. The treatment process involves two stages: irradiation (either gamma irradiation, neutron activation in a nuclear reactor, or electron beam irradiation) to create colour centres in the crystal, followed by annealing (heat treatment) to stabilise the colour. The resulting colour is stable to normal light and heat exposure (GIA; Nassau, K., Gems Made by Man, 1980; AGTA treatment codes).

The commercial consequence: the blue topaz market is one of the most accessible in gemology. Blue topaz of any quality is available at USD 1-15 per carat for most sizes, making it one of the least expensive blue gems. The abundance created by industrial-scale treatment of Brazilian colourless topaz rough has made blue topaz a commodity rather than a gem. It has genuine colour and adequate durability for most jewellery use; it simply has no rarity value at commercial qualities (GIA; Wise, 2016).

Pink topaz

Pink topaz occurs naturally at Ouro Preto alongside imperial orange, and also at Katlang, Pakistan, where fine pink topaz crystals with vivid rose-pink colour are found in pegmatites. Some pink topaz is produced by heating brown or sherry topaz, which converts the brown component and leaves a pinkish tone. Fine natural pink topaz from Pakistan at vivid colour and clean clarity approaches pink sapphire in appearance and commands modest premiums over commercial pink alternatives (GIA; Wise, 2016, pp. 310-313).

Sources

Ouro Preto, Minas Gerais, Brazil: the primary source for imperial topaz in orange, pink-orange, sherry, and pink colours. Colourless topaz from Brazil forms the raw material for the blue topaz treatment industry. Pakistan (Katlang): fine pink topaz crystals. Russia (Ural Mountains): historic source of fine blue and pink topaz. Sri Lanka, Myanmar, Nigeria: minor sources of various colours (GIA; Wise, 2016).

Care: the cleavage warning

The care implication of topaz's perfect basal cleavage is significant enough to warrant a dedicated section. A topaz ring stone set in a prong setting has its table facing up, which is also the cleavage direction. A sharp knock on the table surface, such as hitting a countertop or hard floor with a ring, can cleave the stone cleanly. Bezel settings reduce this risk by providing lateral protection. Ultrasonic cleaners can cause cleavage in topaz if the vibration triggers the cleavage plane. Steam cleaners are not recommended. For any topaz ring intended for daily wear, a protective setting and awareness of impact risk are essential (GIA; Wise, 2016).

Frequently asked questions

Is blue topaz radioactive after treatment?

Neutron-activated blue topaz (the method used to produce the deepest "London Blue" colour) does retain low levels of residual radioactivity after treatment, which is why processed material is tested and held in storage until residual activity decays to safe levels before being released for sale. The regulatory bodies of producing countries (primarily Brazil) and importing countries require testing. Neutron-activated topaz sold in commercial jewellery has been tested and certified safe. The gamma-irradiated and electron-beam-irradiated varieties used for lighter blue shades do not produce significant residual radioactivity. Any blue topaz legally sold in commercial jewellery markets has met the safety requirements of the relevant regulatory authorities (GIA; AGTA; Nassau, 1980).

What is the difference between Sky Blue, Swiss Blue, and London Blue topaz?

These are informal trade designations for different shades of irradiation-treated blue topaz. Sky Blue is the palest and lightest shade, typically produced by gamma irradiation. Swiss Blue is a medium, vivid blue. London Blue is a deep, dark blue-grey, typically produced by neutron activation. All three are irradiation-treated colourless topaz. The colour difference is produced by different irradiation types, doses, and annealing temperatures. None are more natural or more rare than the others; all are commodity material at broadly similar commercial price points (GIA; AGTA; Wise, 2016).

Is topaz a November birthstone?

Topaz is one of the traditional November birthstones in most birthstone lists, alongside citrine. The original November birthstone in earlier lists was yellow topaz specifically (the golden-yellow variety); the modern lists include topaz in any colour. The historical association was with yellow topaz or golden topaz, colours that overlap with imperial topaz's sherry-orange range. Citrine was added as an alternative partly because yellow topaz is rare and citrine provides an accessible alternative in a similar colour range.

Sources cited in this article

  • GIA Gem Reference Guide. (2006). Gemological Institute of America. (pp. 96-99)
  • Nassau, K. (1980). Gems Made by Man. Chilton Book Company.
  • Wise, R.W. (2016). Secrets of the Gem Trade (2nd ed.). Brunswick House Press. (pp. 306-318)
  • Klein, C. (2002). Manual of Mineral Science (22nd ed.). John Wiley and Sons.
  • AGTA treatment disclosure codes (topaz). agta.org.
  • GIA Colored Stone identification. gia.edu.