Ruby
Ruby is corundum that has a purplish- bluish red to yellow-red colour. The finest colour is pure red with a hint of blue, as seen in glowing coal or the red of a traffic light. Burmese "pigeon's blood" rubies possess this fine colour and are the most sought after and expensive of rubies. As the mines in Mogok have been under the strict control of the Myanmar government since 1963, and are leased to nationals of Myanmar only, it is virtually impossible to travel there as a trader. Instead, individuals bring material over the border into Thailand, where towns have been established to accommodate the gem trade.
Vietnam and Tanzania have produced rubies of a similar colour to the pure red of Burmese rubies, but most of them are heated violet and brownish red material. Tanzanian rubies are usually opaque, occurring with green zoisite, and used for decorative carvings. Sri Lankan rubies tend to be light red to, raspberry red in colour. Thai rubies were once dismissed by the trade because of their similarity to garnet, but there is now more Thai than Burmese material produced. The gem-quality Thai rubies have a slightly darker purplish red colour but are "cleaner" stones overall, having less rutile needle inclusions and more lustre than Burmese rubies.
The colour of rubies can be slightly uneven and it is normal for them to have minor inclusions, such as minerals, growth structures, canals and cavities. The inclusions will often indicate the country and area that the ruby came from. For instance, Burmese rubies typically contain rutile, calcite, apatite, olivine, sphene and spinel inclusions. Discoid fractures around natural mineral inclusions often occur and are a sign of heat treatment. Some rubies, in particular Burmese stones, fluoresce under ultra-violet light and good gemstones will even fluoresce in sunlight. However, a useful indicator of synthetic rubies is their very strong fluorescence.
When the rutile needles in a ruby are aligned, asterism occurs in the form of a six-ray star. Ideally, star rubies should be a good translucent red rather than the opaque brownish purple colour usually available. The rays should be sharp silvery white lines that extend to the base of the stone. Rubies may also have a cat's-eye effect, but this is very rare. Trapiche rubies are mainly sought after by collectors and consist of a wheel-like growth of several prismatic ruby crystals. When cut or ground flat, trapiche crystals have the appearance of a wheel with black spokes.
The cutting of rubies usually occurs in the country where they are mined. Pleochroism dictates the orientation of the material, with the table aligned so that the stone has the best red colour. The ruby is then cut, as a step or mixed cut, to yield the greatest weight but not necessarily the best proportions.
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