gemmology.dev – Thailand – Gem Origins Overview

Introduction

Thailand hosts one of the most important gem-producing provinces in Southeast Asia,
centred on Cenozoic alkali basalt fields in the east (Chanthaburi-Trat) and west
(Kanchanaburi). Though primary ruby deposits are largely exhausted, Bangkok and
Chanthaburi remain the world's dominant centres for corundum heat treatment and
the international ruby and sapphire trade.

Geological Context

All Thai corundum deposits share a basaltic origin:

Basaltic Province

Setting: Cenozoic intraplate alkali basalt fields along the Indochina block
Age: Neogene to Quaternary volcanic activity
Transport: Corundum crystallised at mantle depth and was transported to
the surface by alkali basalt magmas
Concentration: Gems accumulate in alluvial and eluvial placers derived
from weathered basalt
Chemistry: Basaltic environment imposes high-Fe, low-Cr signature on
corundum – the defining geochemical contrast with marble-hosted Mogok ruby

Tectonic Context

Post-subduction intraplate extension of the Indochina microplate
Multi-stage sapphire formation at Bo Phloi reflects separate pulses
of basaltic magmatism
Same Southeast Asian alkaline basalt province as Cambodian Pailin field;
deposits merge across the border

Mining Areas

District	Province	Products	Status
Bo Rai	Trat	Ruby (historic 'Siam ruby')	Largely exhausted
Bo Welu / Khlung	Chanthaburi	Ruby, blue/yellow sapphire	Limited activity
Bo Phloi	Kanchanaburi	Blue and yellow sapphire	Mostly closed, well-studied
Kanchanaburi town area	Kanchanaburi	Blue sapphire	Historic

Production History

Thailand's rise and evolution as a gem hub:

Rise to Prominence

1970s–1980s: Thailand became the world's dominant ruby source after Mogok
production declined; Keller (1982) noted Thailand had become "the world's
major source of gem ruby" following "the recent drastic decline in production
from the classic ruby mines of Burma"
Bo Rai ruby production peaked in the 1970s–1980s; now essentially exhausted

Treatment and Trading Hub

Bangkok and Chanthaburi developed into the world's dominant ruby and sapphire
heat-treatment and trading centres
Thailand pioneered the technology of heating Thai and Burmese rough to dissolve
silk and improve colour
Material from Thailand, Cambodia, Vietnam, Burma, and East Africa is routinely
treated and traded through Chanthaburi

Zircon Trade Hub

Beyond ruby and sapphire, Thailand is the global centre for blue zircon production:

Cambodian and Vietnamese zircon rough is imported to Chanthaburi and Bangkok
Heat treatment in oxidising conditions at 900–1000°C converts brownish rough
to the prized "blue zircon" colour
Thailand exports the majority of the world's faceted blue zircon

Thailand – Gem Origins Overview