Siam — The Historical Trade Name for Thai Ruby and Sapphire
Siam — The Historical Trade Name for Thai Ruby and Sapphire
A locality term still encountered in the trade for corundum from Chanthaburi and Bo Phloi
Siam is the pre-1939 name of Thailand and remains in use in the international gem trade as a designation for ruby and sapphire from the Thai-Cambodian deposits at Chanthaburi, Trat, and across the border at Pailin in Cambodia, together with the sapphire-bearing basalt fields at Bo Phloi in Kanchanaburi Province. Siamese material has been a fixture of the international ruby and sapphire trade for the better part of two centuries, and laboratory reports continue to use Thailand or Cambodia as the formal origin term while the trade preserves Siam in catalogue descriptions and historical reference.
Geology and material
The Chanthaburi-Pailin and Bo Phloi deposits are basalt-related, with corundum hosted in alluvial gravels eroded from weathered alkali-basalt flows of late Cenozoic age. This geological setting differs sharply from the marble-hosted deposits of Mogok and Mong Hsu in Burma and the metamorphic deposits of Sri Lanka, and it produces material with characteristic chemical signatures: relatively high iron, low chromium relative to Burmese ruby, and characteristic absence of the calcite, mica, and other inclusions associated with marble-hosted material.
Thai ruby is typically a darker, more brownish or purplish red than fine Burmese ruby, with the high iron content suppressing fluorescence and producing the distinctive sombre hue that the trade has long recognised. Thai blue sapphire from Chanthaburi tends similarly toward a dark, ink-blue colour with strong dichroism; Bo Phloi material extends the species range into yellow, green, and bicoloured (parti-coloured) sapphire that is characteristic of basalt-hosted Australian and Cambodian production.
Bangkok and the cutting trade
Bangkok and Chanthaburi together form one of the world's principal cutting and trading centres for ruby and sapphire, handling not only Thai-origin material but the bulk of African ruby production from Mozambique, Madagascar, and Tanzania, the Burmese trade, and material from Sri Lanka. The historical Thai expertise in heat treatment of corundum — the burning of brown-red Mong Hsu rough into more vivid red, the colour development of geuda Sri Lankan sapphire — has made Bangkok a hub for treated material as well as a market for native Thai production.
In the trade
The term Siam on a stone description should be read in context. On a contemporary laboratory report it means the laboratory has determined a Thai or Thai-Cambodian origin and is using the historical trade name; on antique jewellery descriptions it indicates the historical period when most coloured stones reaching the European trade from Southeast Asia were assumed to be Siamese; on dealer parlance for high-iron, sombre-coloured ruby it is a stylistic reference rather than a strict origin claim. Buyers should ask which usage applies. Pricing on Thai-origin ruby and sapphire reflects the typically darker colour of basalt-related material and runs at a discount to comparable Burmese or Mogok material.