Corundum of a red colour are identified as rubies. Being corundum these have all the optical, physical and chemical properties of the mineral. These also have negligible quantities of chromic oxide in their compositions which do not in any way affect the general chemical formula. It is this element, chromic oxide which is responsible for the red colour of ruby. The colour in rubies could be in various shades and tones and is dependent on the quantity of chromic oxide present, its ratio in relation to other elements in the composition, and the nature of its distribution. The depth and intensity of color is related to these factors while the distribution of colour in ruby could be patchy, banded or uniform. The presence of chromic oxide in the correct proportions, in the desired quantities, in the correct manner of distribution within a glassy textured transparent body, free from flaws and inclusions produced the ruby of optimum quality. Such stones are most beautiful and are highly prized. In Sri Lanka the occurrence of rubies are comparatively rare in comparison to blue sapphires although adequate supplies are available to cater to the market. Superlative quality rubies from Miyanmar (Buruma) hace earned universal acclaim as the best in comparison to rubies from other regions of the world. This statement does not in any way express the idea that there is no exception to the rule. Other localities have occasionally shown up rubies of exceptional beauty that compare well with or even surpass the beauty of their Myanmar counterparts. Sri Lanka has proved this point although such occurrences have been rare. Such stones have always been from the Uda – Walawe region, which can be considered as the locality for rubies that compare most favorably with the best from Miyanmar.
Determining the possible locality of a ruby by colour and appearance alone would certainly lead to confusion and controversy. To eliminate such controversy other factors like mineral inclusions and twinning planes are of importance and should be taken into consideration. Rubies are generally sensitive to light purely because the sensitivity of the colouring element, chromium; and their luster appearance could differ according to the nature of the light under which they are observed. It is best that rubies are observed under natural daylight conditions, always remembering the fact that daylight conditions are different in different parts of the world; which factor could consequently creat a different colour sensation on the human eye.
The colour in rubies could vary from pale to a deep and intense red; the colours ranging from pale rose red to a deep red. Most Sri Lankan varieties are of a pinkish red and display a tinge of purple which factor perhaps is sufficient to betray to the experienced person that the stones are of Sri Lankan origin. These purplish tints are attributed to the presence of iron in addition to chromic oxide in the composition. Such stones when subject to intense heat would either lose or diminish the purplish tint thereby highlighting the principle colour, red. In his book “precious Stones”, Bauer (1971) states that “ the shade of colour most admired is the deep pure carmine red with a slightly bluish tine.” It is not uncommon to notice this faint bluish tinge in this category of rubies. This colour is referred to as ‘pigeon blood red’ in gem circles. Furthermore, the colour distribution in rubies is invariably either patchy or banded with colour bands alternating with the colourless. The patchy colour distribution in the ruby could be remedied by subjecting such stones to intense heat. It has been noted that the colour in such stones when gradually heated gets distributed as a final outcome. Flawless rubies of a deep intense red with uniform distribution of colour and with a pleasing tone are extremely rare and consequently very valuable.
A currently prevailing controversial question is ; where lies the demarcation line between the ruby as defined and the pink sapphire? Difference of opinion is rife. But sooner or later this issue has to be resolved. In this publication it is not intended to discuss this subject in detail.