The Burmese ruby
The Burmese ruby is by far the rarest and most valuable of the corundum stones and although the colours range from pinkish red to dark red the finest examples are termed pigeon blood red. Indeed it is the colour that is the principle determiner of the value of a ruby. Rubies have been mined in Burma (Myanmar) for at least five centuries and today gemstones are the country’s fifth largest export product by value. Decades of military rule and endemic corruption has resulted in an industry that is grossly under-recorded and vast quantities of stones are smuggled across the border into China. The most famous ruby mining areas in Burma are Mogok in the north and Mong Hsu approximately 150 miles to the east of Mandalay. Akin to emeralds, rubies generally have significant inclusions and eye clean (let alone loupe clean) specimens are extremely rare. For this reason very high clarity rubies often turn out to be synthetic and there are many setting techniques that have been developed to minimise the visibility of inclusions within the stones and deepen their colour. Consequently visual inspection of loose stones is a far better indication of their quality compared with assessments made after they have been set.
(HM Queen Elizabeth II wearing a fine collection of Burmese rubies)
As discussed in earlier articles, it is important to remember that both sapphires and rubies hail from the corundum family of gemstones and the only thing that separates them is their colour (sapphires encompass all non-red corundum precious gems). Pure corundum is actually colourless and (as with all coloured gems) it is impurities of trace elements that give them their colour. At 9 on the Moh’s hardness scale, both rubies and sapphires are only surpassed in hardness by diamonds, making them ideal for jewellers to work with. Most gems are formed of a crystalline structure, consisting of multiple planes of symmetry which are grouped into seven systems according to the number of axes, their length and angles of intersect. Rubies and sapphires share a trigonal crystal structure due to the fact they have three planes of symmetry and four axes, where three of these axes are set at 60 degrees to one another in the same plane. The fourth axis is perpendicular and unequal in length to the other three. Burmese rubies are said to have a glassy or vitreous lustre, which determines the amount of light reflected at the surface of the stone.
(Moh’s scale of hardness)
The chemical composition of corundum is aluminium oxide and it is the presence of trace impurities, including chromium, iron, vanadium and titanium, that give rise to the spectrum of colours in which they naturally occur. The red colouration in rubies is largely as a result of the presence of chromium. Even very slight variation in the depth of colour can have a profound effect on the value of a Burmese ruby and the optimum colour is a highly saturated medium to medium dark red. Those stones that are significantly darker or lighter will fetch a much lower price. Although rubies are found in a number of locations, including Burma, Africa, Thailand and Cambodia, and they all contain the same chemical properties, their colour differs greatly and the Burmese stones are at the top of the pyramid. Burma rubies reflect red to pinkish red in most types of light, whereas Thai and Cambodian stones tend to be darker and more brown in appearance. Burmese rubies are most commonly found as round, oval, cushion or emerald cut stones, due to the normal shape of the rough. Some rubies are cut into a cabochon shape in order to diffuse the light entering them in such as way as to refract it from the fine inclusions of rutile within the stone to generate a star effect. This star usually has 6 points but highly skilled cutters can create a 12 pointed, or double star, although this is extremely rare and very highly sought after.
(A very rare double star ruby)
Over 90% of precious corundum has received some form of treatment or enhancement. Most typically this involves heat treating the stones and it is accepted practice, providing the seller discloses the fact that it has been carried out, and it results in improved transparency and colour. Many of these techniques are very simple and can involve just placing the stones in an open fire but some sophisticated approaches have been developed in which pressures and exact temperatures are used to maximise the benefits. The result of treatment is permanent and these stones do not require any ongoing special care or attention. Lower grade rubies are often fracture filled with lead glass in order to give the outward appearance of greater clarity and or colour and the resulting stone is inherently unstable. Some experimental treatments are being perfected in which corundum are treated with heat-diffusion techniques which involve the addition of foreign elements to adapt the colour to the desired one. Stones that have been certified by a respected gem lab will display the fact they have been treated and the type of treatment or enhancement that has been employed.
Rubies from Mogok attract such a level of interest due to the purity and saturation of the red colour coupled with a strong fluorescence under UV light which has the effect on enhancing the ‘glow’ of the stones in natural light. It is these two factors combined that generate the sumptuous red allure of the Burmese ruby. This has resulted in the ruby entering the mythology of many civilisations throughout the ages. In the Hebrew bible and the Quran Aaron, the older brother of Moses, was said to have had a ruby placed around his neck when God created 12 gemstones, making the ruby the most precious of them all. The bible also states that Moses was given the Ten Commandments by God on tablets of sapphire and as such corundum became gemstones of ‘divine favour.’ It is no surprise that corundum stones have taken pride of place among the crown jewels of many royal families, including those of the United Kingdom.