Thailand is both a gem source and one of the most important gem trading and treatment centers in the world. Its own deposits, mainly around Chanthaburi and Trat in the southeast and Bo Phloi in Kanchanaburi, produced large amounts of ruby and sapphire, especially in the second half of the 20th century. Even more lasting is the country's role as the place where rough from Asia and Africa is heated, cut and traded, centered on the gem markets of Chanthaburi and Bangkok.
Basalt-related corundum deposits
Thai ruby and sapphire come from a different setting than the marble-hosted rubies of Mogok in Myanmar. In eastern and western Thailand, young alkali basalt lavas erupted through older rocks and carried corundum crystals up from deep in the crust or upper mantle. As the basalts weathered, the hard corundum crystals collected in soils and stream gravels, forming alluvial deposits.
Rubies from basalt-related deposits tend to contain more iron than marble-hosted rubies. Iron dulls fluorescence, so Thai rubies usually glow less under ultraviolet light, and they often look deeper, darker red, sometimes with a purplish or brownish tint. Blue sapphire from basalt sources is often dark and inky as well. Both ruby and sapphire are forms of the mineral corundum.
Chanthaburi and Trat
Chanthaburi province, in southeastern Thailand near the Cambodian border, and the neighboring province of Trat were Thailand's main ruby and sapphire fields. Gem mining in the area goes back a long way, and miners and traders from Burma, including Shan people, played a large part in its growth from the 19th century.
The Chanthaburi and Trat fields produced a large share of the world's commercial ruby for several decades in the mid and late 20th century. Mining was a mix of small hand pits and larger mechanized operations that dug and washed big volumes of gravel. By the late 20th century the richest deposits had been heavily worked, and output fell. Over the same period, land use changes and environmental controls limited new mining in many places.
Chanthaburi did not lose its importance when local mining slowed. The town's gem market, held in streets lined with dealers' offices, became one of the busiest colored stone trading markets in the world. On market days, buyers sit at tables while brokers bring parcels of rough and cut stones from many countries.
Bo Phloi sapphire in Kanchanaburi
Bo Phloi, in Kanchanaburi province in western Thailand, is the other well-known Thai corundum field. Like the eastern fields, its sapphire comes from weathered basalt. Bo Phloi has been known mainly for blue sapphire, along with black star sapphire and dark blue to greenish stones. Large-scale mining operated there in the late 20th century.
Bangkok and the heating revolution
Thailand's biggest influence on the gem world is heat treatment. Heating corundum to improve color and clarity is an old practice, but Thai burners took it much further from the 1970s onward. They found that heating pale, milky Sri Lankan "geuda" sapphire to very high temperatures could dissolve its fine rutile silk and turn it a rich blue.
That discovery turned a low-value by-product into a major source of fine-looking blue sapphire. Thai traders bought large amounts of geuda rough from Sri Lanka and treated it at home. Thai burners also developed methods for removing purple tones from rubies and improving stones from many other sources.
Later developments came out of the Thai trade too. In the early 2000s, beryllium diffusion treatment appeared in the market, producing strong orange, yellow and padparadscha-like colors in sapphire, and its detection became a major task for gem labs. Lead-glass-filled rubies, which use glass to fill fractures in very low-grade material, also appeared in the trade in this period. The pages on ruby treatments and sapphire treatments explain each process and how it is disclosed.
Cutting and trading
Bangkok and Chanthaburi cut huge numbers of colored stones. Thai cutters are known for skill with corundum and for producing mixed cuts that keep weight while showing good color. Cutters also handle rough from Madagascar, Mozambique, Tanzania, Myanmar, Sri Lanka and Australia.
Bangkok has a large gem and jewelry district around Silom Road and Mahesak Road, with dealers, cutting workshops, jewelry factories and gem testing laboratories. The city hosts a major gem and jewelry trade fair and is home to several gemological labs and schools. Its jewelry industry exports set pieces as well as loose stones.
The rise of Thailand as a colored stone center parallels the shift of diamond cutting to Gujarat. For an overview of cutting cities, see great gem cutting centers.
Buying gems in Thailand
Thailand's busy trade also means buyers should be careful. Tourist scams involving overpriced or misrepresented gems have been widely reported in Bangkok for decades, often starting with a friendly stranger steering visitors to a shop. Reputable dealers disclose treatments, and stones of high value should come with a report from an independent lab. See how to buy a gemstone for general advice.
Frequently asked questions
Does Thailand still mine rubies?
Thailand's main ruby fields around Chanthaburi and Trat were heavily worked through the 20th century, and output fell sharply by its end. Today Thailand matters far more as a trading, cutting and treatment center than as a source.
Why are Thai rubies darker than Burmese rubies?
Thai rubies come from basalt-related deposits and usually contain more iron. Iron reduces red fluorescence and tends to give a deeper, sometimes purplish or brownish red.
Why is Thailand important for sapphire heating?
Thai burners developed high-temperature heating of pale Sri Lankan geuda sapphire from the 1970s, turning it deep blue. Thailand remains a center for corundum treatment and for the trade in treated stones.
Is Chanthaburi a good place to buy gems?
Chanthaburi has one of the largest colored stone markets in the world, but it is a dealer's market. Buyers without trade experience should rely on reputable sellers and independent lab reports.