Topaz occurs in the colours blue, blue green, brown, yellow, sherry yellow and pink. Red is extremely rare. It is said that pink as a natural occurrence. is also rare and that most of the pink stones available in the market today are invariably those that have been subjected to heat treatment. The dark yellow and the brownish stones from certain regions of the world when subject to slow heat could be transformed into pink. The colourless variety is the most abundant in Sri Lanka and the world over. As the gem communities of the world are on the constant lookout, mainly for coloured varieties of gemstones with the exception of Diamonds, the more common colourless gemstones were not considered important. As such only a small quota of the available natural colourless material is cut and polished for gemstones. This factor coupled with its very abundance has made Topaz a cheap commodity. Nevertheless colourless Topaz when absolutely clear, together with its characteristic high luster and ability to take on and retain a high degree of polish makes it a useful stone in jewellery. Its luster is sometimes so good that certain records have drawn a parallel to diamonds by making reference to these as ‘slaves’ diamonds. In Sri Lanka the only colour varieties of topaz available are blue, blue green and off colourless varieties. All these are comparatively rarer than the colourless varieties. Colourless Topaz is found both as rolled water worn pebbles and as ‘in situ’ occurrences in several parts of the country, particularly around Polwatte in the Matale- Rattota area, Balangoda region, and the recently discovered regions around Sigiriya.
In the recent past the technique of inducing colour to colourless topaz by a process of irradiation developed rapidly. For this purpose different procedures have been adopted: - (i) gamma ray irradiation (ii) subjection to high energy electrons by the use of linear accelerators or (iii) with neutrons in neutrons in nuclear reactors (Schmetzer, 1987) Consequent to irradiation the clear colourless topaz takes on beautiful shades of blue. It has been found that topaz from Sri Lanka are the most susceptible to such treatment and produce the best results in respect of colours, as these have minimum impurities. What is more the Sri Lankan material has been found to 'cool off' in the least possible period of time after irradiation, thus reducing the radiation levels sufficiently for use as ornaments. These new irradiation possibilities generated a boom time bonanza for Sri Lanka and the country engaged itself in a very profitable industry processing and exporting topaz for irradiation purposes (Irradiation for colour enhancement of topaz is not carried out in Sri Lanka). Through these processes of irradiation three to four colour shades of blue are produced. As already stated the Sri Lankan material has been found to produce the best colours and this has been attributed to the fact that these have less impurities in comparison to those of other localities. It could also be said that this may be due to the variable fluorine-hydroxyl ratio differences in the composition of topaz from different localities.
Topaz is a fluo-silicate of aluminium combined with small amounts of ferrous oxide, lime, alkalies and water (hydroxyl). The water present is capable of replacing a certain amount of fluorine and this could cause slight variations in the optical and physical properties like the refractive indices and density, in stones of different localities. Accordingly the density could vary between 3.53 and 3.56. Its refractive indices could be 1.61-1.62 or 1.63-1.64, and its double refraction is 0.010. Topaz has a hardness of 8 in Mohs's scale. This mineral crystallizes in the orthorhombic system and has a marked cleavage in the direction of the basal plane. It has been noticed that minute fissures could develop on the cleavage planes and these minute fissures could produce brilliant iridescence which affect the transparency and clarity.
Of the colour varieties of topaz, yellow is said to be the most typical. Among the blue varieties, pure blue, greenish-blue and bluish varieties are known. The hues are invariably pale, making the occasional dark blue something very special. A very close observation of the stones will more often than not reveal a tinge of green. The pale colours could resemble aquamarine.
Impure colour, fissures in the cleavage planes, turbidity, cavities, both liquid and gaseous are common in practically all topaz.