Topaz (gem)

Species: TOPAZ

Variety: According to the colors

Color: All colors

Transparency: Transparent

Crystal system: Rhombic

Chemical formula: A12SiO4(F,OH)2

Chemical composition: Aluminum fluorosilicate

Refraction: Positive biaxial birefringent

Refractive index: 1.618-1.621-1.628

Birefringence: 0.010 (-0.002)

Dispersion: 0.014

Pleochroism: T.pink and red: weak to strong, light red and yellow; t.brown, yellow and orange: weak to strong, yellow-brown, yellow and yellow-orange; t.violet: weak to strong, blue-violet and red-purple; t.green: weak, green-blue and light green

Fluorescence: T.colorless, blue and green: generally none, sometimes yellow or green faint (OL) and very faint (OC); other colors: generally yellow-orange faint (OL) and very faint (OC)

Density: 3.53

Hardness: 8

Habitus: Pseudotetragonal prismatic

Genesis of deposits: Pneumatolitic magmatics

Main deposits: Algeria, Australia, Brazil (Minas Gerais, Minas Novas), England, Japan, Mexico, Myanmar, Namibia, Nigeria (Jos), Pakistan (Gilgit), Russia (Siberia, Urals), Scotland, Sri Lanka (Matale), United States (California, Colorado, New Hampshire, Texas, Utah), Zimbabwe (Miami)

Technical specifications

The name topaz probably derives from the island of Topazos in the Red Sea: according to some, the etymology is instead to be referred to the Sanskrit term tapas (fire). Topaz crystals are found mainly in veins of pneumatolitic magmatic origin in rocks rich in silica. Given the high hardness, it is possible to obtain a well-preserved mineral even in secondary deposits of alluvial formation. Topaz crystallizes in the rhombic system and belongs to the nesosilicates, that is, its structure is based on isolated tetrahedra SIO, linked by aluminum in octahedral coordination to which the F and OH ions are also linked. This very compact structure is the cause of the high density and hardness: in fact, topaz was chosen by Mohs as the eighth term of his scale. The habitus is prismatic pseudotetragonal with crystals elongated along the c-axis terminating at the ends with the combination of pyramids and a basal pinacoid. The cleavage is perfect according to the basal plane. The fracture is conchoidal and the luster is vitreous. Topaz should not be subjected to high temperatures because it could form fractures as well as color variations. Some brown stones can discolor with prolonged exposure to sunlight. Polishing should not be done with ultrasound or steam but only with warm water and soap.

The many colors in which this mineral is found determine the varieties that must be indicated by following the term "topaz" with the color of the sample being examined (e.g. yellow topaz, blue topaz, colorless topaz, etc.). Any other terminology must be abandoned. The yellow-orange variety is the best known and most appreciated in jewelry: it was known by the incorrect name of "imperial topaz" and is still imitated by citrine quartz. In the past, the blue variety was considered equally valuable, with shades ranging from intense blue to blue-green: in the last decade, significant quantities of treated material have been placed on the market, indistinguishable from natural, which has caused a dramatic drop in price. Topaz can take on pink tones while red coloration is extremely rare. Colorless topaz was used as an imitation of diamond even if the only characteristics that the two minerals have in common are color and density. The other varieties are rarely used in jewelry.

The nature of the colors is to be attributed to color centers of unknown origin for the blue, brown and yellow varieties, and to Cr3+ ions in octahedral coordination for the pink and red colors, while the other colors are presumed to be formed by the union of the causes already mentioned (e.g. violet = red + blue, green = blue + yellow, orange = red + yellow). The varieties of topaz can be divided into two groups characterized by slightly different refractive index and density values.

Generally, there are colorless, green and blue varieties with n = 1.608-1.611-1.618 and density from 3.53 to 3.57; the yellow, brown, orange, pink, red and purple varieties usually have n = 1.628-1.631-1.638 and density from 3.49 to 3.53. It is not always possible to verify the biaxial character of this gem with a refractometer because the intermediate index 𝞫 is close to the value of the minor index 𝞪. The displacement of the minor index therefore appears very limited and is not always detectable; in this case the sample may appear to be positive uniaxial. Fortunately, there are no positive uniaxial gems with indices close to those of topaz and, in any case, this can be distinguished from tourmaline, brazilianite, apatite, andalusite and damburite by the higher density values. Very rarely, topaz can exhibit chatoyancy in blue and yellow-orange colors.

Internal features

Samples used in jewelry generally do not have many inclusions and in some cases may even be free of them. Liquid inclusions in topaz tend to take the form of negative crystals arranged on planes parallel to the c-axis and the faces of the prism. Fluid inclusions often have the appearance of two-phase levels but are actually formed by two immiscible liquids such as aqueous saline solutions and liquid carbon dioxide. Rarely, a solid phase may also be present, which is usually made up of rock salt, quartz, sylvite or cryolite. Solid inclusions include: prismatic apatite crystals; acicular or prismatic plagioclase crystals; tufts of goethite, yellow or red if altered; sheets of muscovite mica, colorless; crystals of fluorite and monazite; needles of tourmaline or hornblende; sheets of hematite that may alter to limonite.

Treatments

By irradiating pink, red, purple or colourless topazes it is possible to obtain brown-orange varieties. By heating it is possible to obtain the opposite effect, going from brown to colourless samples or to pink, if chromium ions are present. Recognizing these treatments is difficult and not always possible. The pink colour obtained by heating shows a much stronger pleochroism (light pink - yellow), compared to the natural one, which is rare. Brown topazes obtained by irradiating colourless ones have the typical refractive indices of colourless material, lower than the natural brown variety. Some brown topazes can discolour with exposure to sunlight while others have a stable colour. This occurs due to the presence of different colour centres, stable and unstable, whose nature is still unknown. By irradiating colorless topazes, brown-green varieties are also obtained, whose yellow component is due to unstable color centers that are subsequently deactivated by moderate heating or simply by exposure to sunlight. Once the yellow component is eliminated, blue, light blue or light-green topazes are obtained, stable to sunlight, identical to those found in nature. By heating to about 450°C, both natural and treated blue topazes fade and become colorless again. Recognizing this treatment is practically impossible.

Even if particularly intense blue tones, not normally found in nature, reveal the artificial origin of the color, there are no commonly used and non-destructive instrumental tests capable of proving this fact. It must also be considered that naturally blue gems may have undergone, in the millennia following their formation, a similar process of natural irradiation. The radiation used for the "bombardment" of topazes are usually gamma rays, generated by the radioactive isotope cobalt 60; these radiations produce uniform coloration, do not require the consumption of electrical energy and limit to a minimum the possibility of inducing radioactivity in the treated sample. X-rays do not have sufficient energy to activate the color centers while neutrons and electrons can induce residual radioactivity in the gems. The colorless topazes used for the treatment are generally low in impurities at the atomic level; this is an advantage since some impurities could be "activated" during the process, in turn generating radioactivity for long periods.