A Mantle-Tapping Eruption
The sapphires of Chanthaburi owe their existence to a specific type of volcanic activity involving alkali basalts. These dark, fine-grained volcanic rocks erupted from deep within the Earth's upper mantle. Geologists have determined that the source of these Cenozoic-era eruptions lies near the transition zone with the crust, at depths of around 80 kilometers. The basaltic magma ascended rapidly, carrying for pre-existing crystals it encountered on its ascent.
These gems are not native to the basalt they are found in; instead, they are classified as xenocrysts, a term meaning "foreign crystals". The sapphires formed under immense pressure and temperature deep within the crust or upper mantle long before the volcanic eruptions that brought them to the surface. The basaltic magma simply picked them up and carried them. Several factors support this conclusion: corundum (the mineral name for sapphire) is chemically unstable in basaltic melt, and many of the mineral inclusions found within the sapphires could not have formed in an alkali basalt environment.
Radiometric dating of the basalt flows in the Chanthaburi area, specifically at a location called Khao Wua, indicates the volcanic activity occurred around 3.0 million years ago, during the Early Pliocene epoch. Over millions of years, the hot and humid climate of Thailand has caused the relatively soft basalt to weather away into a reddish mud, leaving behind concentrations of the much harder and more durable sapphires in residual soils and alluvial deposits.
A Sapphire's Fingerprint
Chanthaburi sapphires are a form of the mineral corundum (Al₂O₃). Their desirable blue color comes from the presence of trace amounts of iron and titanium within the crystal lattice. The area also produces green, yellow, and black star sapphires. Gem mining in this region, particularly at a site known as Khao Ploi Waen ("Hill of Gems"), was first reported in 1850.
Microscopic examination of these sapphires reveals a internal inclusions that identify the source. Common mineral inclusions are plagioclase feldspar, zircon, pyrochlore, and monazite. The presence of zircon is particularly useful for geologists. Zircon crystals contain uranium, which decays to lead at a known rate, allowing scientists to use U-Pb dating methods to determine the age of the crystals and, by extension, the geological events associated with them.
For centuries, miners used traditional methods, digging vertical shafts down 7 to 8 meters to reach the gem-bearing gravel layers. While most of the major deposits were exhausted by the 1980s through more mechanized mining, small-scale operations continue today. The town of Chanthaburi transformed into a global hub for the gem trade, for local stones and gems from around the world, due to the local expertise in heat treatment developed to improve the color and clarity of the sapphires. Today, it is estimated that 70% of the world's sapphires pass through Chanthaburi for treatment and trading.