gemmology.dev – Vietnam — Luc Yen Ruby and Cobalt-Blue Spinel

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 in the
Red River shear zone yielded gem-quality corundum and spinel in marble, first
reaching international buyers after Vietnam's 1987 economic reforms.

Luc Yen ruby is low-Fe, Cr-dominant, and shows strong red LWUV fluorescence; its
primary diagnostic inclusions are nordstrandite (aluminium hydroxide, exceptionally
rare as a gem inclusion globally) and pyrrhotite (iron sulfide), documented by Kane
et al. (1991). Vietnam also hosts the world's leading commercial source of cobalt-blue
spinel: Co²⁺ substitution in the spinel structure produces three distinctive UV-Vis
absorption bands and a colour unmatched by any other major deposit. Luc Yen spinel
is additionally separated from Mogok and Kuh-i-Lal by titanite inclusions and
trace-element profiles in Ni, Zn, and Ti. ^[1]^[2]

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 ^[3]
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 ^[4]
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) ^[1]; 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" in a marble metamorphic genesis ^[2]
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) ^[3]
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

Vietnam — Luc Yen Ruby and Cobalt-Blue Spinel