Russian Demantoid Garnet – Ural Mountains
Ural demantoid (andradite, Cr-coloured); horsetail inclusions – byssolite terminology and Kissin 2021 mineralogical correction; LA-ICP-MS origin discrimination from Namibia and Madagascar.
Introduction
Russian demantoid garnet from the Ural Mountains is the rarest and most prized
variety of andradite garnet and among the most valuable of all garnets. Discovered
in the 1860s from alluvial placers along the Bobrovka River (tributary of the Sysert),
it is coloured by Cr³⁺ and possesses the highest dispersion (0.057 B–G interval)
of any natural garnet – exceeding diamond (0.044). The name derives from German
"Demant" (diamond), referring to this exceptional fire.
Discovery and History
Ural demantoid history:
- The name "demantoid" was proposed in 1856 by Finnish mineralogist Nils von
Nordensheld referring to diamond-like fire; confirmed as andradite garnet in 1874 - Commercial gem-quality crystals were recovered from alluvial gravels of the
Bobrovka River (Sysert area, Sverdlovsk Oblast) beginning in the 1860s - Additional primary occurrences at Poldniovaya, Karkodino, and Nizhny Tagil district
- Late 19th century Russian demantoid became fashionable in European jewellery
(Fabergé pieces); stones command significant market premiums over Namibian
or Malagasy material
Properties
| Property | Value |
|---|---|
| Species | Andradite garnet – Ca₃Fe₂(SiO₄)₃; Cr³⁺ substituting Fe³⁺ |
| Crystal system | Cubic (isometric); singly refractive |
| RI | 1.880–1.895 |
| Dispersion (B–G) | 0.057 – highest of all natural garnets |
| SG | ~3.84 |
| Hardness | 6.5 (Mohs) |
| Colour | Yellow-green to emerald-green (Cr³⁺ dominant) |
| Key absorption | 685 nm Cr line; 440 nm blue-violet cutoff (iron) |
| Fluorescence | Inert (iron quenches) |
The Horsetail Inclusion – Dual Terminology
The horsetail is the most diagnostic inclusion for Ural demantoid:
FGA Traditional Description (Byssolite / Asbestiform)
- FGA examination terminology: The "horsetail" inclusion consists of curved,
fan-shaped bundles of fine asbestiform fibres radiating from a central chromite
crystal – described in the gemmological tradition as "byssolite" (asbestiform
actinolite/amphibole variety) - This is the terminology used by major gemological laboratories in origin
reports and is the description that FGA Diploma candidates must recognise - Source: Phillips & Hyrsl (1996), "Russian Demantoid, Czar of the Garnet
Family," Gems & Gemology – API-confirmed [VERIFIED]
Mineralogical Correction (Kissin 2021)
- Kissin, Murzin, and Karaseva (2021) performed SEM, Raman, XRD, and thermal
analysis on the inclusion structure and found: "in most cases, 'horsetail'
inclusions in the Ural demantoid were represented by hollow channels and only
the outcrops, on the demantoid surface, were occasionally filled with serpentine" - The fibres are primarily HOLLOW GROWTH CHANNELS; serpentine-phase fill occurs
only at crystal surface outcrops – not byssolite or chrysotile sensu stricto
throughout the inclusion body - Source: Kissin et al. (2021), Minerals, doi: 10.3390/min11080825 – API-confirmed [VERIFIED]
Horsetail Terminology – Important Clarification
Diopside Needles (Secondary Inclusion)
A secondary inclusion type also confirms Ural origin:
- Diopside needles: Krzemnicki (1999) confirmed by Raman microspectroscopy
that "diopside needles" are present as inclusions in Russian demantoid – distinct
from the fibre horsetail; elongated, colourless to pale green - Combined with the horsetail, diopside needles reinforce Ural provenance
- Source: doi: 10.5741/gems.35.4.192 [VERIFIED]
Origin Discrimination by LA-ICP-MS
| Feature | Ural (Russia) | Namibia (Erongo) | Madagascar |
|---|---|---|---|
| Horsetail inclusions | Present (chromite core) | Absent | Absent |
| Diopside needles | Occasional | Absent | Absent |
| Host rock | Serpentinite | Skarn | Skarn |
| Mn/Ti ratio | Low | Intermediate | Higher |
| Wollastonite, fluorapatite | Absent | Present | Present |
| Diopside grains | Needles (elongated) | Absent | Rounded groups present |
Chemical Discrimination Note
Schwarzinger (2019) demonstrated that three major demantoid sources (Russia, Namibia,
Madagascar) can be distinguished by LA-ICP-MS: "the three major sources could be
distinguished by using a plot of the manganese/titanium ratio versus the sum of
chromium and vanadium in combination with the aluminum content." This chemical
separation supplements the inclusion-based diagnosis.
Market Position
Ural demantoid in the gem market:
- Commands significant premium over Namibian and Malagasy material – attributable
primarily to the horsetail inclusion and historical prestige - Fine Russian demantoid with prominent horsetail, good green colour, and >1 ct
weight is among the most collectible of all garnet varieties - Supply is limited; alluvial placers are largely worked out; primary occurrences
yield small quantities - Namibian demantoid (skarn-hosted, Erongo) is the principal commercial alternative
for clean, larger stones