Vietnam – Luc Yen Ruby and Cobalt-Blue Spinel
Marble-hosted ruby and spinel from Luc Yen (Yen Bai) and Quy Chau; low-Fe high-Cr chemistry, cobalt-blue spinel, distinction from Burmese material.
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vietnam luc-yen yen-bai quy-chau ruby spinel marble-hosted cobalt-blue origin/vietnam
Introduction
Vietnam's northern gem provinces rank among the world's most important marble-hosted
corundum and spinel sources. The Luc Yen district (Yen Bai Province) and Quy Chau
(Nghe An Province) produce ruby and spinel in a geological setting analogous to
Mogok, Burma: Himalayan-related metamorphism of carbonate-platform sequences yielded
gem-quality corundum and spinel in marble. Vietnam also hosts the world's leading
source of cobalt-blue spinel – a unique and highly prized material.
Geological Context
Marble-hosted gem formation in northern Vietnam:
Luc Yen District
- Located in Yen Bai Province; the carbonate platforms were subjected to
"intense metamorphism during successive orogenies" in the Red River shear zone - Marble-hosted genesis analogous to Mogok (Burma): gems crystallised in
metamorphosed limestone during Himalayan-related orogenic events - Produces ruby, pink spinel, blue spinel, sapphire, moonstone, and tourmaline
Quy Chau
- Central Vietnam; Nghe An Province
- First deposits opened to international buyers after Vietnam's economic
reform (doi moi) in 1987 - Marble-hosted ruby; historically the entry point for Vietnamese rubies
into the international market
Ruby – Luc Yen and Quy Chau Types
Characteristics of Vietnamese marble-hosted ruby:
Colour and Appearance
- Vivid pinkish-red to red; can approach Mogok quality
- Often lighter and more saturated than Thai basaltic ruby; some with
slight pinkish or purplish modifiers - Typically lower clarity than Mogok material – much production is cabochon
or star quality
Diagnostic Inclusions
- Calcite rhombs: Marble-hosted environment; characteristic of all
marble-type corundum globally - Pyrrhotite: Iron sulfide crystals – highly diagnostic for Luc Yen
- Nordstrandite: Rare aluminium hydroxide mineral; documented in Luc Yen
ruby by Kane et al. (1991) – very unusual in corundum globally - Bluish colour zones and angular growth features: Characteristic of
Vietnamese marble ruby - Fluid inclusions: CO₂-rich primary inclusions with multi-solid residues;
brine compositions differ from Mogok in specific salt assemblage
Spectroscopy and Fluorescence
- LWUV fluorescence: Strong red – critical contrast with Thai/Cambodian
basaltic rubies where iron quenches fluorescence - Low Fe: Marble-hosted chemistry; typically <300 ppm Fe by LA-ICP-MS
- High Cr: Dominant chromophore; similar principle to Mogok
Nordstrandite and Pyrrhotite
Distinguishing Vietnam from Burma (Mogok)
| Feature | Luc Yen (Vietnam) | Mogok (Burma) |
|---|---|---|
| Fe content | Low (<300 ppm) | Low (<300 ppm) |
| LWUV fluorescence | Strong red | Strong red |
| Key inclusions | Pyrrhotite, nordstrandite, calcite | Calcite, apatite, sphene, silk |
| Ga/Mg ratio | Relatively higher Ga | Lower Ga – lab criterion |
| Colour zones | Bluish zones, angular features | Irregular; treacle swirls |
| Fluorescence nuance | Strong red (similar to Mogok) | Strong red (benchmark) |
| Lab separation | LA-ICP-MS Ga/Mg + inclusions | Reference standard |
Spinel – Cobalt-Blue Luc Yen Type
Vietnam's most celebrated and distinctive gem material:
Cobalt-Blue Spinel
- Luc Yen is the world's leading source of vivid blue spinel coloured by Co²⁺
- Co²⁺ substitution is rare in spinel globally; most blue spinel is Fe-coloured
- Chauviré et al. (2015) established that the blue "is due to cobalt (Co²⁺),
with some iron contribution" – a marble metamorphic genesis - UV-Vis spectroscopy: Cobalt produces three characteristic absorption bands;
the Co²⁺ spectrum is distinctive from Fe-coloured blue spinel
Red and Pink Spinel
- Full colour range: vivid red, pink, orange, lavender, purple
- Trace element variation within Luc Yen: Cong Troi sub-deposit has low Zn
(<500 ppm); An Phu spinels are Zn-rich (up to 11,000 ppm) - Titanite inclusions and dislocation systems are Luc Yen-specific features
that differ from Mogok (apatite, calcite, octahedral negative crystals) and
from Kuh-i-Lal/Tajikistan (different Ti, Ni, Zn, Sn profiles)
Spinel Origin Determination
Separating Luc Yen spinel from Mogok and Kuh-i-Lal:
- Co²⁺ blue: Confirmed by UV-Vis and LA-ICP-MS cobalt content; no other
major spinel source produces commercially significant cobalt-blue material - Trace element profile: Ti, Fe, Ni, Zn, Zr, Sn ratios differ between
Luc Yen, Mogok, and Kuh-i-Lal; Malsy & Klemm (2010) demonstrated this separation - Titanite inclusions: Diagnostic for Luc Yen; absent in Mogok and Kuh-i-Lal
- Mn/Ti plot + Cr+V: Chemical discrimination from Mn/Ti vs Cr+V is applicable
to spinel from these Himalayan-belt sources