Pailin District, Pailin Province, Cambodia
The Pailin field is the main corundum-bearing region in the west of the country near the town of Pailin, Battambang Province, it is the eastern continuation of the important sapphire and ruby bearing region of Chanthaburi-Trat in adjacent Thailand. It was discovered in the Dangrek Range in 1874 and has been operated more or less continuously; except for wars and revolutions.Alluvial deposits in streams and small rivers, which erode weathered basalts and ancient sandy-boulder-clay alluvium, are of the most economic importance (Jobbins and Barrange, 1981). Gems concentrate in the lower part of a 0.5 to 1 m thick sandy-pebble-gravel layer. The heavy fraction of this layer contains zircon, densely coloured, brown-red pyrope, black spinel, magnetite, ilmenite, olivine, as well as sapphire and ruby.
Ruby prevails in the eastern part of the Pailin field (Phnom O Tang and Phnom Kho Ngoup), while sapphire is predominant in the western part on the Phnom Yat lava plateau.
Pailin sapphires have pale blue to dense blue colours. Rubies are pink to violet-red and brown-red, the bright red "pigeon's blood" is rare. Colourless, yellow and green stones are almost absent. The refractive index is 1.760 - 1.770, density is 3.993 - 4.007 g/cm³.
Most of the Pailin corundum is heat-treated to improve the transparency and colour.
Ref: E.Ya. Kievlenko (2003) Geology of gems, pp. 46, 67
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