Thai Sapphire – Bo Phloi and Kanchanaburi Types
Basaltic high-Fe blue sapphire from Bo Phloi and Kanchanaburi; strong 450/460/470 nm iron triplet, common heat treatment, basalt-suite inclusions.
Introduction
Thai sapphire from the Bo Phloi district (Kanchanaburi Province) represents the
classic basaltic, high-iron corundum type of western Thailand. The deposits were
mined through much of the twentieth century and are scientifically well characterised;
commercial production is largely concluded but the material is a standard reference
for basaltic sapphire identification in gemmological education. [1]
Diagnostic significance mirrors Thai ruby: the basaltic environment imposes a high-iron,
high-titanium trace-element profile. The resulting spectroscopic signature (a strong
450/460/470 nm iron absorption triplet) is the most reliable single indicator; it
produces the dark, steely or inky blue tone and suppresses UV fluorescence to inert or
very weak. [2] Basaltic inclusions (zircon with radiation
halos, nepheline, hercynitic spinel, ilmenite, enstatite) confirm geological parentage
and are absent in Kashmir or Ceylon material.
Heat treatment is the norm; virtually all commercial material has been treated. After
heating, the trace-element signature and fluorescence inertness remain diagnostic even
when original silk inclusions have dissolved.
Colour Range
Thai basaltic sapphires display a wider colour range than marble-hosted sapphires:
- Blue: Predominant; dark to very dark blue-green; often described as "inky"
- Yellow: Common in Bo Phloi; golden to greenish-yellow
- Green: Blue-green to green from high Fe content
- Black star: Star sapphire with opaque body
- Golden star: Asterism in yellowish material
The very dark tone of Thai blue sapphires – often darker than Kashmir or Ceylon –
results from the combined effect of high-Fe absorption across the visible spectrum.
Diagnostic Inclusions
Khamloet et al. (2014) identified two groups of mineral inclusions from Bo Phloi: [1]
Felsic Alkaline Suite
- Alkali feldspar (orthoclase/sanidine)
- Nepheline (feldspathoid)
- Sapphirine (deep crustal / high-P indicator)
- Biotite-phlogopite mica
Contact-Metamorphic Suite
- Hercynitic spinel (dark, opaque)
- Zircon crystals – common; often with radiation damage halos
- Manganiferous ilmenite (black opaque)
- Silica-rich enstatite (pyroxene)
- Almandine-pyrope garnet
- Staurolite
- Calcite (occasional – from xenolith material)
- Monazite (rare, distinctive)
Inclusion Significance
Spectroscopic Properties
Characteristic UV-Vis features of Thai blue sapphire:
Iron Triplet
- Strong absorption at approximately 450, 460, and 470 nm (Fe²⁺–Ti⁴⁺ charge
transfer + Fe–Fe intervalence) - This triplet is significantly stronger than in Kashmir or Ceylon sapphire
- The triplet contributes to the "steely" or "inky" dark tone
- Similar in strength to Cambodian material but distinct from Kashmir's velvety blue
Origin Determination by LA-ICP-MS
Chemical separation of Thai sapphire from other basaltic sources:
- High Fe, high Ti: Primary geochemical signature of basaltic parentage
- Ga/Mg ratio: Basaltic sapphires have elevated Ga and different Mg profiles
vs marble-hosted material - Fe/Ti discrimination: Separates Thai from Cambodian (subtle difference),
and both from Kashmir, Ceylon, and Australian - No rutile silk of the Kashmir type
- No calcite of the marble-hosted type
Heat Treatment
References
- ↑ 1. Khamloet, P.; Pisutha-Arnond, V.; Sutthirat, C. (2014). Mineral inclusions in sapphire from the basalt-related deposit in Bo Phloi, Kanchanaburi, western Thailand. Russian Geology and Geophysics. DOI: 10.1016/j.rgg.2014.08.004.
- ↑ 2. Palke, A.; Renfro, N.; Berg, R. (2019). Geographic Origin Determination of Blue Sapphire. Gems & Gemology, 55(4), 536–579. DOI: 10.5741/gems.55.4.536.