The king of gemstones, synonymous with hot, seductive red stands for passion since time immemorial. The fabled chromian red variety of corundum, gem grade Aluminium Oxide of mostly metamorphic origin, has been celebrated since antiquity. World class Rubies easily rank amongst the most valuable gems in the world.
Ruby at its best displays either a pure vivid red or a red with perhaps a maximum of five to fifteen percent of purplish modifier infusing a deep richness into the overall body colour. Widely referred to as ‘Pigeon’s Blood’ this shade is traditionally associated with the finest Burmese rubies. Stones of latter origin are often supercharged in saturation due to red fluorescence which may express itself freely in the absence of the trace element Iron. Iron is known to quench fluorescence and brightness.
Burmese stones also regularly contain fine silk which increases colour refraction without reducing transparency. Red rubies with a very minor touch of orange modifier follow. Although less appreciated by the market, some experts do prefer them as orange arguably reinforces the dominant Red bodycolour. Finally, Pinkish Red to Pink-Red Rubies follows in value. Despite being slightly less saturated, they remain highly popular with the ladies, particularly in western countries as well as Korea, Japan. Vietnam’s Luc Yen is a classic source of fluorescent pinkish Rubies, somewhat similar to classic Burmese material.
The optimal tone for ruby is a medium to the medium dark of approximately 70 to 80 % saturation for rubies at which red reaches its greatest intensity. Since rubies are pleochroic changing from an orangy Red to purplish Red depending on the orientation of the crystal, a good cutter has to ensure a stone shows the right face-up colour. Nonetheless, a forgivable slight multicolour effect is often seen, which usually is aligned parallel to the crystal axis. This has to be distinguished from ‘Bleeding’, a distinctly negative effect in which colour loses its saturation when a stone is shifted from natural to artificial light.
Clarity is the next most important valuation factor. Since clean rubies are rare, significant premiums are paid for impeccable stones. However tiny rutile (titanium oxide) silk is beneficial towards light scattering and as long as it does not affect the crystal much, can add a ‘velvet softness’ and beauty to stone as opposed to total transparency which may appear too ‘glassy’. Given so few rubies are clean, a few eyes visible yet well-concealed inclusions are acceptable.
The most famous sources are Mogok, Burma and Thailand (‘Siam’) which hold a certain pedigree. Truly world class stones also have been found at Luc Yen, Vietnam, Didy, Madagascar and Winza, Tanzania. Stones from the latter find feature a particularly fine crystal, often surpassing Burmese stones. Although Burma is unrivalled at its best, spectacular stones are very difficult to come by nowadays and ruby lovers are advised to opt for these other sources as true beauty knows no origin. Other noteworthy occurrences are Mong Hsu (Burma), Longido (Tanzania) known for pure deep reds, Tajikistan (pinkish Red), Sri Lanka (of a distinct purplish Red).
Starrubies are an unusual variety in which rutile causes asterism. Stones are typically cabbed as should be judged equally for colour and transparency. The best have sharp somewhat modest stars hovering above a beautiful bodycolour of red at high translucency. They remain more of collector’s stones in the west, although they are highly regarded in Asia.
Lastly rubies are commonly heated, with traditional heat treatment (mentioned as ‘H’ on certificates) for fine stones being tolerated by the trade. High-temperature heat treated stones, often mentioned as H(a) on certificates or worse H(b) or H(c) stones should best be avoided if lasting value is desired. These may contain anything from insignificant residues as in H(a), to minor residues H(b), and obvious residues including glass like ‘fillers’ in H(c). Here the question arises where the trade and the consumer should draw the line.