Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al₂SiO₄(F, OH)₂. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces. Topaz actually has an exceptionally wide color range that, besides brown, includes various tones and saturations of blue, green, yellow, orange, red, pink, and purple. Colorless topaz is plentiful, and is often treated to give it a blue color. Topaz is also a pleochroic gemstone and can have varied color intensity when viewed at different angles. Topaz is a hard and durable gemstone, and will not dissolve in most chemical solvents. However, it does have perfect cleavage which can make it prone to chipping or forming flaws if banged hard. Due to its good cleavage and pleochroic nature, care must be exercised when faceting Topaz gemstones.
History of topaz in Sri Lanka
Sri Lanka is historically known as the cradle of treasures for its precious, natural gemstones and it is a world celebrated destination for mining and faceting valuable colored gems including Ceylon Blue Sapphire, rubies, garnets, and alexandrite. It has been experienced that with high prices of sapphires and other precious stones traded in the market, all consumers are not been able to afford blue sapphires. So as a next best alternative to blue sapphires, Topaz has been chosen as a much cheaper gemstone with good properties of hardness and lustre.
In the recent past, the technique of inducing colour to colourless topaz by a process of irradiation developed rapidly. 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. So, to take advantage out of this, a contract has been offered by America to the gem and jewellery authority for topaz to be cut in Sri Lanka and sent for the treatment.
Gemstone mining as a traditional industry in Sri Lanka runs way back to the era of ancient kings. The traditional, environment friendly and ethical mining methods practiced under the regulatory supervision of the National Gem and Jewellery Authority (NGJA) have set many benchmarks in the international Gemstone Mining Arena. The most commonly used Gemstone mining method in Sri Lanka is Pit-Head Mining and Tunneling. Surface Placer Mining and River Bed Dredging are the other methods used widely.
Mainly, Topaz crystals are found on land areas and rounded to semi-rounded stones are found in the deposits of paddy fields. Matale district of Sri Lanka comprises of a fairly large number of deposits of industrial minerals and precious and semi precious minerals and the district is famous for pegmatite deposits that generates good quality topaz. Excellent outcrops in Matale area provide unique inghts into the evolution of hydrothermal and pegmatite rocks in the central highlands in Sri Lanka.
Topaz is also found in Sigiriya, Balangoda and Nawalapitiya areas. Mines in these areas are excavated by companies after they receive permission from the Gem and Jewelry authority of Sri Lanka. Usually, open pit mining has been carried out by almost all the mining areas.
The cutting process
In the recent past the technique of inducing colour to colourless topaz by a process of irradiation developed rapidly. Since Sri Lanka has mainly white topaz they will be sent to other countries for irradiation treatment. Two main ways that could be sent for this treatment are,
- Cutting the stones and polishing them first and then sending for the treatment.
- Slicing the rough stones and then sent for treatment, finally cut them once they are back from the treatment.
The 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. Normally it takes around 1 – 2 years for the treatment to be completed, but 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 these material has been found to ‘cool off’ in the least possible period of time after irradiation, thus reducing the radiation levels. Through these processes of irradiation three to four colour shades of blue are produced.
To get the lutreous blue topaz color there should be some treatments done since original blue topaz colour doesn’t occur in the gemstones. These treatments are,
- Heating – Which is used to change the yellow and brown topaz to pink color.
- Coating – Which is produce a wide range of colors but doesn’t give good color and gets the coating removed very easily.
- Irradiation – Topaz can be irradiated to enhance, change or deepen their color. This process can take place in a nuclear reactor (neutron bombardment), an accelerator (electron bombardment), or by exposure to gamma rays in an irradiator. Although Topaz is mined in its colourless form, a radiant Swiss blue, London blue and sky blue colours can be rendered by irradiation.
Processes of irradiation –
Large quantities of colorless topaz are currently being treated by gamma irradiation to turn them blue, a process that does not produce radioactivity in the stone. Colorless topaz turns brown with gamma ray treatment, and is then subsequently heated to blue. The resulting color is a medium deep blue that is significantly darker than that of most natural blue topaz. Aconsiderably darker color does result in rare instances. Gems are exposed to radiation emitted by Cobalt-60 or Cesium-137, both highly radioactive isotopes (or “radioisotopes”) of these two elements. These materials are sealed within stainless steel rods for safety, and release gamma rays during radioactive decay.
Linear Accelerators (linacs):
This uses powerful electromagnets to activate a beam of electrons which creates clour when they pass through the gem. These electrons can create residual radioactivity when high energies are used. These accelerators are known as a preferred choice for gems that require high doses of radiation, which includes, topaz and yellow beryl. Also, As the electrons pass through the gem there will be substantial heat created, which requires the gem to to be cooled during treatment to prevent any damage.
These reactors create the most intense colours of topaz. By having vessels holding uranium fuel rods, nuclear fission in the reactor produces high energy neutrons and gamma rays that create colour as they pass through the gem. The bombardment that takes place can create residual radioactivity. This is the main method used to treat Sri Lankan stones. These Lankan stones are mainly sent to countries such as America, Germany and Poland for this treatment.
Results after irradiation
After the Irradiation process three types of Blue Topaz are obtained.
1)Sky Blue Topaz – A lighter version of the Blue/ Similar to the colour of sky.
2) Swiss Blue Topaz – For this color to be produced the stone should be fully clean and doesn’t not have any inclusions. If there are inclusions, the stone may get damaged during the process. This is one of the most desired colours of blue topaz. Baby swiss is a bit lighter than the swiss blue topaz and some consumers demand this colour in markets nowadays.
3) London Blue Topaz- When a stone has inclusions this colour will be given. This is a darker version of blue topaz.
The below table shows diffrerent aspects of blue topaz during the process and its main markets.