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 nesosilicate minerals sharing the cubic crystal system but varying
in chemical composition across two main series: pyralspite (aluminium garnets: pyrope,
almandine, spessartine) and ugrandite (calcium garnets: uvarovite, grossular, andradite).
All are singly refractive (isotropic), with no pleochroism, and RI ranging from 1.73 for
pyrope to 1.89 for andradite; values that reliably distinguish garnets from birefringent
species such as tourmaline or peridot. Hardness is 6.5–7.5 Mohs and SG 3.50–4.30
depending on species. [1] The group covers virtually every colour except blue,
though rare pyrope-spessartine colour-change garnets exist. Commercial importance ranges
from ubiquitous red almandine (Fe²⁺, RI ~1.79) to exceptional demantoid andradite (the "king of garnets") whose dispersion of 0.057 exceeds diamond's 0.044 and whose
Russian Ural specimens with diagnostic "horsetail" byssolite inclusions command prices
rivalling fine emerald. [2] Vanadium-chromium grossular
(tsavorite) from Kenya and Tanzania offers an intense green that rivals emerald without
the need for fracture-filling treatments. [3] [4]
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 [1]
| 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 [1]
- 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 [1]
- 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:
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
Horsetail Inclusions
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 [4]
- 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 |
References
- ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. Phillips, M.; Talantsev, A. (1996). Russian Demantoid, Czar of the Garnet Family. Gems & Gemology, 32(2), 100–111. DOI: 10.5741/gems.32.2.100.
- ↑ 3. Belley, P. (2024). Age Determination of Gem-Quality Green Vanadium Grossular (Var. Tsavorite) from the Neoproterozoic Metamorphic Mozambique Belt, Kenya and Tanzania. Crystals, 15(1), 9. DOI: 10.3390/cryst15010009.
- ↑ 4. Fritsch, E.; Rossman, G. (1987). An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions. Gems & Gemology, 23(3), 126–139. DOI: 10.5741/gems.23.3.126.
- ↑ 5. Kovalchuk, O. (2021). "Horsetail" Inclusions in the Ural Demantoids: Growth Formations. Minerals, 11(8), 825. DOI: 10.3390/min11080825.