gemmology.dev – Thai Ruby – Chanthaburi-Trat and Bo Rai Types

Introduction

Thai ruby from the Chanthaburi-Trat province and adjacent Bo Rai (Trat Province)
represents the classic "Siam ruby" of the gem trade. All Thai ruby is basalt-hosted:
gems crystallised at mantle depth and were transported to the surface in alkali
basalt magmas, concentrating in alluvial placers. The high-iron basaltic environment
distinguishes Thai ruby sharply from low-iron marble-hosted Mogok material.

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)

Fe content: High (>600 ppm; often >1,000 ppm) – primary criterion
Fe/Cr ratio: High relative to marble-hosted ruby
Ga/Mg: Basaltic sapphire signature; specific ratio differs from marble-hosted
Low Cr relative to Fe: Weak chromium signal suppressed by high iron

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

Inclusion Criteria

Basalt-xenolith mineral suite: zircon, ilmenite, enstatite, feldspar, sapphirine
ABSENCE of calcite, apatite, sphene, pargasite
Coarse, irregular rutile networks rather than fine short Mogok-type silk

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