Thai Ruby – Chanthaburi-Trat and Bo Rai Types
Basaltic-hosted Siam ruby from Chanthaburi-Trat and Bo Rai; diagnostic high-Fe chemistry, 451/460/470 nm iron triplet, weak fluorescence, alluvial habit.
Introduction
Thai ruby from the Chanthaburi-Trat province and adjacent Bo Rai district represents
the classic "Siam ruby": alluvial, basalt-hosted corundum that filled the supply gap
when Mogok production declined in the 1960s–1970s. [1]
All Thai ruby crystallised at mantle depth and was transported by Cenozoic alkali basalt
magmas; water-worn, rounded rough habit is itself a secondary indicator of alluvial
concentration.
The defining diagnostic is the iron absorption triplet at approximately 451, 460, and
470 nm in the UV-Vis spectrum, a very strong signal from Fe²⁺–Ti⁴⁺ intervalence charge
transfer in the high-iron lattice. [2] This triplet absorbs
blue-violet light, producing the brownish modifier that distinguishes Thai ruby from the
pure chromium red of Mogok. Long-wave UV fluorescence is weak to inert.
Bo Rai deposits are largely exhausted; most material described as "Thai ruby" today is
rough from Cambodia, Vietnam, or East Africa heat-treated in Chanthaburi. [3]
Origin determination must separate geological origin from treatment location.
Colour and Appearance
Characteristic appearance of Thai basaltic ruby:
Colour
- Hue: Red to slightly purplish-red; often darker than Mogok material
- Brownish modifier: Common due to high iron content; iron absorbs in the
blue-green region and shifts the colour toward brownish-red - Saturation: High in fine material but rarely achieves the vivid "pigeon
blood" quality of Mogok ruby - Fluorescence response: Colour appears dull or dead under UV – iron
strongly quenches chromium fluorescence
Transparency and Cut
- Typically water-worn, rounded alluvial pebbles – evidence of secondary deposit
- Variable clarity; extensive rutile silk common
- Most commercial material is heat-treated to improve colour and clarity
The Iron Absorption Triplet
Diagnostic Inclusions
| Inclusion | Description | Significance |
|---|---|---|
| Zircon crystals | Common; often with radiation damage tension halos | Basaltic parentage marker |
| Ilmenite | Manganiferous; black opaque | High-Fe basaltic environment |
| Enstatite | Silica-rich; pyroxene group | Diagnostic for xenolith-origin basaltic ruby |
| Alkali feldspar | White to colourless inclusions | Deep crustal/mantle xenolith suite |
| Almandine-pyrope garnet | Rounded crystals | Xenolith association |
| Sapphirine | Blue-green; rare but diagnostic | High-P basaltic xenolith environment |
| Biotite-phlogopite mica | Brown platelets | Basaltic mantle xenolith |
| Rutile silk | Coarse, irregular networks | Differs from fine Mogok silk |
| Calcite / apatite / sphene | ABSENT | Presence would indicate marble-hosted origin |
Inclusions Note
Origin Determination Criteria
Laboratory criteria for confirming Thai basaltic ruby origin:
Chemical Criteria (LA-ICP-MS)
Optical / Spectroscopic Criteria
- Very strong 451/460/470 nm iron absorption triplet in UV-Vis
- LWUV fluorescence: Weak to inert (iron quenches chromium fluorescence),
a major contrast with marble-hosted ruby - Chromium doublet at 692/694 nm present but may be accompanied by strong
broad iron absorption - Chelsea filter: May appear weakly red (Cr present) but less vividly than
Mogok material
Comparison with Mogok Ruby
| Feature | Thai (Basaltic) | Mogok (Marble-hosted) |
|---|---|---|
| Fe content | High (>600–1,000 ppm) | Low (<300 ppm) |
| LWUV fluorescence | Weak to inert | Strong red |
| 451/460/470 nm triplet | Very strong | Absent or very weak |
| Colour modifier | Often brownish-red | Pure red / pigeon blood |
| Key inclusions | Zircon, ilmenite, enstatite | Calcite, apatite, sphene, silk |
| Silk character | Coarse, irregular networks | Fine, short rutile needles |
| Geological host | Cenozoic alkali basalt | Precambrian marble |
| Market premium | Lower | Highest |
Market and Treatment
Thai ruby in the modern gem trade:
- Heat treatment: Nearly universal for commercial material; dissolves silk,
improves colour; readily accepted and disclosed - Unheated Thai ruby: Uncommon; little premium (basaltic character remains diagnostic)
- Market position: Below Mogok, Mozambique primary; valued for volume supply
- Trade history: Bo Rai deposits largely exhausted by 1990s; most "Thai ruby"
in trade today originates from Cambodia, Vietnam, or East Africa, heated in Thailand [1]
References
- ↑ 1. Shor, R.; Weldon, R. (2009). Ruby and Sapphire Production and Distribution: A Quarter Century of Change. Gems & Gemology, 45(4), 236–259. DOI: 10.5741/gems.45.4.236.
- ↑ 2. Palke, A.; Renfro, N.; Berg, R. (2019). Geographic Origin Determination of Ruby. Gems & Gemology, 55(4), 580–613. DOI: 10.5741/gems.55.4.580.
- ↑ 3. Promwongnan, S.; Sutthirat, C. (2019). Mineral Inclusions in Ruby from Bo Welu, Chanthaburi, Thailand. Gems & Gemology, 55(3), 354–371. DOI: 10.5741/gems.55.3.354.