Another trading centre for gemstones was in Goa. Many merchants and miners went there to sell the best they had, for they were given full liberty to sell whereas in their own countries, they were compelled to show their output to the kings and princes and sell gems at whatever price the rulers ordered.
During the nineteenth century, Crawfurd (1827), G,d'Amato (1833), Oldham (1855) and Bredermeyer (1868), among others, have reported in detail on the Burmese mines. The rubies produced were generally very small, less than 0.25 rati because Chinese and Tartar merchants had smuggled out the larger pieces. The few big stones they left behind were flawed and valuable stones were rare. A full account of the ruby mines in Burma is also given by Scott and Hardiman in the Gazetteer of Upper Burma and the Shan States, Rangoon of 1901 and the Imperial Gazetteer of the same year. Dr Ball locates the principle mines in three valleys, Mogok (Mogout), Kathe and Kyatpyen. The elevated tract including these valleys was situated at a distance of about one hundred and forty kilometres from Mandalay at an elevation of about eighteen hundred metres above sea level. The ruby tract covered a very beautiful but malarial area of twenty-six sq kilometres with the mining in around two thirds of this area. Sagyin, only twenty five kilometres from Mandalay, had also some ruby mines, but the stones obtained there were of inferior quality.
Several other gemstones were found in this mining area, sapphires, emeralds, amethyst, topaz, white sapphires, spinel, zircon (hyacinth), iolite and rubellite (a variety of tourmaline). Only about one per cent of the corundum was ruby. Caves in the marble hills of the area contained ruby clay from which the finest stones were extracted.
Located in thick jungle, pure red rubies from the Mogok Stone Tract in Burma, one of the richest gem bearing areas of the world, are now in constant demand. Ruby and sapphire deposits lie within thirty metres below ground and Burmese ruby miners sift out gravel obtained by digging short tunnels into the hillside where deposits have been located. When deposits are found in old river beds, small tunnels of about ten metres are dug and the watery sludge is panned to retrieve the heavier gem concentrates. Final sorting is by hand. Large mining enterprises carry out open cat mining by eroding the rock and earth with strong water jets. The wet slurry is then washed and sieved to separate the gemstones. Such mining leaves the dug out area in the form of terraces.
Corundum has a specific gravity of 4.0, a refractive index of around 1.765 and a hardness of 9 on the Moh's scale (on this scale, diamond is the hardest substance with a value of 10). The elementary crystal is barrel shaped, with six oxygen atoms surrounding each aluminium atom. Because of this complex crystal structure, ruby and sapphire cannot be cleaved or split like a rough diamond and cutting or sawing may often cause the stone to crack during grinding. While it is being polished overheating often leaves parallel cracks that run from facet edges.
The impurity elements that colour corundum are usually iron, chromium, titanium and vanadium in concentrations of a few parts per million. When they occupy interstitial sites displacing the aluminium atoms, these impurities cause defects in the crystal lattice-this can produce extra energy levels in the crystal which the takes on a colour that ranges through pink, blue, yellow and orange, the varied hues of sapphire and ruby.
The purity of the colour of a sapphire depends greatly on the concentration of iron as an impurity. If only iron is present, the stone assumes a yellow tinge that is in great demand in India and is known as 'Pushkraj'. The presence of vanadium and titanium atoms in close proximity within the aluminium oxide crystal causes a deep blue colour in the stone- termed 'Neelam' in India. When only Chromium is present as an impurity, corundum becomes a pure red ruby through pink and violet sapphires indicate the presence of chromium in the stone. The Chromium content in ruby usually varies from 0.05 per cent in the palest pink stones to about 0.5 per cent in the deepest red, at which point the colour saturates. Black sapphire is coloured by an admixture of magnetite, hematite or spinel. The value of corundum depends mostly on its colour and clarity. The best corundum does not have much 'fire' or lustre, but blue pure yellow sapphire and the red ruby are the most valuable in this category.