|Chanthaburi Sapphire. Credit: Vincent Pardieu-GIA|
Thailand has been a leader in the global corundum trade since the 1950s. Bangkok and Chanthaburi remain important treatment and trading centers even though many—though not all—of the country’s corundum deposits are nearly exhausted.
Since the early 2000s, there’s been a small-scale mining revival in Thailand’s Chanthaburi province. Chanthaburi’s sapphire deposits are associated with alkali basalts, where the gems occur as xenocrysts in basalt in the Khao Ploy Waen and Bang Kacha areas. Local farmers partner with miners to work the sapphire deposits and return the land back to agricultural use when mining ceases.
A new report from the GIA Laboratory in Bangkok, ‘A Study of Sapphire from Chanthaburi, Thailand and its Gemological Characteristics’ presents their findings. 59 blue/greenish-blue sapphire samples obtained from a miner in the Khao Ploy Waen area, Chanthaburi, were fashioned to optimize them for LA-ICP-MS chemical analysis, UV-Vis-NIR and FTIR spectroscopy, and inclusion studies using microscopy and Raman spectroscopy.
Thai sapphires resemble gems from other basalt-related deposits in Cambodia, Nigeria, Cameroon, Australia, Laos and Northern Madagascar. As sapphires from these sources share similar geologic origins, it’s challenging to discriminate between them. Magnification revealed angular growth structures, needles, and particles, along with mineral inclusions like plagioclase feldspar, zircon, pyrochlore, monazite, mica, and molybdenyte.
UV-Vis-NIR and FTIR spectra show features typical of basalt-related sapphires. Although LA-ICP-MS analysis showed a large overlap with the trace element chemistry of sapphires from other basalt-related deposits, the authors identify some overall trends that could prove helpful. They also suggest a combination of techniques—trace-element chemistry and microscopy—can help separate Thai blue sapphire from Cambodian and Nigerian samples with similar characteristics. However, the authors note to further support geographic origin determination, the relationship between inclusions, spectra and chemistry in basalt-related sapphires needs further analysis and study.
The above story is based materials provided by GIA.