Garnet Group
The garnet group minerals including almandine, pyrope, spessartine, grossular, andradite, and their gem varieties like demantoid and tsavorite.
Introduction
Garnets are a group of silicate minerals sharing a common crystal structure but
varying in chemical composition. The group produces gems in virtually every colour
except blue, from the common red almandine to the rare and valuable demantoid.
Understanding garnet requires recognizing it as a mineral group, not a single species.
Most gem garnets are solid solutions between end-member compositions.
The Garnet Group
Garnets divide into two main series that can intergrade:
Pyralspite Series (Aluminium Garnets)
| Species | Formula | Colour |
|---|---|---|
| Pyrope | Mg₃Al₂(SiO₄)₃ | Red to purplish-red |
| Almandine | Fe₃Al₂(SiO₄)₃ | Red to brownish-red |
| Spessartine | Mn₃Al₂(SiO₄)₃ | Orange to reddish-orange |
Ugrandite Series (Calcium Garnets)
| Species | Formula | Colour |
|---|---|---|
| Uvarovite | Ca₃Cr₂(SiO₄)₃ | Emerald green (rare as gems) |
| Grossular | Ca₃Al₂(SiO₄)₃ | Colourless to green to orange |
| Andradite | Ca₃Fe₂(SiO₄)₃ | Green to yellow to black |
Physical Properties
| Property | Value Range |
|---|---|
| Crystal system | Cubic (isometric) |
| Hardness | 6.5-7.5 Mohs |
| Specific gravity | 3.50-4.30 (varies by species) |
| Refractive index | 1.73-1.89 |
| Optic character | Singly refractive (often shows ADR) |
| Pleochroism | None (isotropic) |
| Lustre | Vitreous to resinous |
Anomalous Double Refraction
Important Gem Garnets
Almandine
The most common garnet:
- Colour: Deep red to brownish-red to purplish-red
- Chromophore: Iron (Fe²⁺)
- RI: ~1.79; SG: ~4.05
- Spectrum: Three characteristic Fe bands at 505, 520, 575nm
- Sources: Worldwide; India, Sri Lanka, Brazil, USA
Pyrope
Magnesium-rich garnet, often from volcanic sources:
- Colour: Blood red to purplish-red
- Chromophore: Iron, with chromium in some
- RI: ~1.74; SG: ~3.75
- Sources: South Africa, Arizona, Czech Republic (historic)
- Note: "Bohemian garnet" was historically prized pyrope
Rhodolite
A pyrope-almandine intermediate with distinctive colour:
- Colour: Purplish-red to raspberry
- Composition: ~70% pyrope, 30% almandine
- RI: ~1.76; SG: ~3.84
- Sources: Sri Lanka, Tanzania, Brazil, Madagascar
- Market: Popular for its attractive, lighter red-purple colour
Spessartine
Manganese garnet with vibrant orange colours:
- Colour: Orange to reddish-orange ("fanta" orange)
- Chromophore: Manganese (Mn²⁺)
- RI: ~1.80; SG: ~4.15
- Sources: Nigeria (fine orange), Namibia, Brazil, Madagascar
- Note: "Mandarin" or "Fanta" garnet refers to vivid orange spessartine
Grossular Varieties
Grossular produces several important gem varieties:
Tsavorite
Green grossular, one of the most valuable garnets:
- Colour: Vivid green (chrome/vanadium coloured)
- Chromophore: Vanadium (V³⁺) and/or chromium (Cr³⁺)
- RI: ~1.74; SG: ~3.61
- Sources: Kenya, Tanzania (East African Rift)
- Rarity: Fine stones over 2 carats are rare
- Named: By Tiffany's Henry Platt after Tsavo National Park
Hessonite
Orange to brown grossular ("cinnamon stone"):
- Colour: Orange to reddish-brown to yellow-brown
- Characteristic: "Roiled" or "heat wave" internal appearance
- RI: ~1.74; SG: ~3.65
- Sources: Sri Lanka, India, Brazil, Canada
- Note: Internal "treacly" appearance is diagnostic
Mali Garnet
Grossular-andradite mix from Mali:
- Colour: Yellow to yellowish-green
- Properties: High RI (~1.77), good dispersion
- Source: Mali, West Africa (discovered 1990s)
- Note: Higher dispersion than pure grossular
Andradite Varieties
Andradite includes the most valuable of all garnets:
Demantoid
Green andradite, the "king of garnets":
- Colour: Vivid green to yellowish-green
- Chromophore: Chromium (Cr³⁺)
- RI: ~1.88; SG: ~3.85
- Dispersion: 0.057 (higher than diamond!)
- Spectrum: Cr line at 443nm ("horse line")
- Sources: Russia (Ural—finest), Namibia, Madagascar
Horsetail Inclusions
Russian demantoids are prized for "horsetail" inclusions:
- Byssolite (fibrous amphibole) radiating from a point
- Considered diagnostic for Ural origin
- Presence adds value (unusual for inclusions)
- Non-Russian demantoid typically lacks horsetails
Topazolite
Yellow andradite:
- Colour: Yellow to brownish-yellow
- Sources: Italy, Switzerland, USA
- Note: Collector interest; not commonly seen in jewellery
Melanite
Black andradite:
- Colour: Black (titanium-bearing)
- Uses: Victorian mourning jewellery
- Sources: Italy, Germany, Mexico
Demantoid Value
Colour-Change Garnet
Some garnets show alexandrite-like colour change:
- Type: Usually pyrope-spessartine mixes
- Change: Blue-green (daylight) to red-purple (incandescent)
- Cause: Vanadium and/or chromium
- Sources: Madagascar, Tanzania, Sri Lanka, USA
- Rarity: Exceptionally rare; highly valued
Identification Notes
Key features for garnet identification:
Distinguishing Species
| Test | Use |
|---|---|
| Refractive index | Separates species within range |
| Specific gravity | Variable; helps narrow possibilities |
| Spectrum | Almandine Fe bands diagnostic |
| Colour | Initial indicator but not definitive |
| Inclusions | Horsetails in demantoid; roiled in hessonite |