Garnet is the trade name for a group of related silicate minerals that share a common cubic crystal structure and a general formula of A3B2(SiO4)3, where A is a divalent cation (calcium, magnesium, iron, or manganese) and B is a trivalent cation (aluminium, iron, or chromium). The substitutions across these sites produce a continuous range of compositions and a remarkable range of colours, refractive indices, and optical effects. Garnet is one of the most varied gem groups in the trade, and modern mineralogy recognises six principal end-members and many intermediate compositions.

The end-members and their gem expressions

The six principal end-members are pyrope (Mg3Al2(SiO4)3), almandine (Fe3Al2(SiO4)3), spessartine (Mn3Al2(SiO4)3), grossular (Ca3Al2(SiO4)3), andradite (Ca3Fe2(SiO4)3), and uvarovite (Ca3Cr2(SiO4)3). Most natural garnet is intermediate in composition between two or more of these end-members, and trade names often refer to specific intermediate ranges.

Pyrope-almandine series produces the deep red garnets historically called Bohemian garnet (when from the Czech sources) or American garnet (when from the United States). The composition is typically pyrope-dominant with iron substitution producing the characteristic red colour. Refractive index ranges from approximately 1.74 to 1.78 and specific gravity from 3.74 to 4.10 across this series.

Almandine-spessartine series produces colours from purplish-red through orange-red to orange. Spessartine itself, when of pure composition and bright orange colour, is the variety often marketed as mandarin garnet, with the finest material from Loliondo in Tanzania, from Madagascar, and from the Kunene region of Namibia. Spessartine refractive index runs higher, approximately 1.79 to 1.81, and specific gravity 4.12 to 4.20.

Pyrope-spessartine intermediate compositions produce the variety known as Malaia or Malaya garnet, mined in the Umba Valley of Tanzania, with colours ranging from peach through reddish-orange to brownish-red. The colour of Malaia garnet often shifts perceptibly between incandescent and daylight illumination, and individual stones may show genuine colour-change behaviour. Pyrope-spessartine with chromium substitution produces colour-change garnet that shifts from blue-green or grey-green in daylight to purplish-red or red in incandescent light, with stones from East Africa, Madagascar, and Sri Lanka in the trade.

Grossular produces multiple gem varieties. Tsavorite, the chromium- and vanadium-coloured green grossular discovered in Kenya in 1967 and in Tanzania shortly after by Campbell Bridges, is the most highly priced grossular variety, with fine material approaching the green of Colombian emerald and reaching prices in excess of ten thousand dollars per carat for stones over five carats. Hessonite (cinnamon stone) is the orange to brownish-orange grossular, with characteristic heat-wave inclusion patterns and a distinctive treacle-like internal appearance. Hydrogrossular, a hydrous variety with replacement of some silica by hydroxyl, occurs as a translucent material from the Transvaal in South Africa, sometimes marketed as Transvaal jade.

Andradite produces demantoid, the chromium-coloured green variety with the highest refractive index (1.880 to 1.889) and dispersion (0.057) of any natural garnet. Demantoid from the Ural Mountains of Russia, discovered in 1868, is the historical type and is characterised by horse-tail inclusions of byssolite (chrysotile asbestos). Demantoid from Namibia, discovered in 1996, lacks horse-tail inclusions and tends to a slightly more yellowish-green colour. Topazolite is the yellow andradite, melanite the opaque black andradite. Andradite generally has lower hardness (6.5 to 7) than other garnets (7 to 7.5), and demantoid in particular requires care in setting.

Uvarovite, the chromium end-member, occurs almost exclusively as small bright green crystals in chromite deposits of the Urals, Outokumpu in Finland, and a few other localities. Crystals are rarely large enough to facet, and uvarovite reaches the trade primarily as druzy specimens.

Optical and physical properties

Garnet is a singly refractive (isometric) mineral, distinguishing it under polariscope from doubly refractive species. The refractive index varies across the group from approximately 1.73 (pure pyrope) to 1.89 (pure andradite). Specific gravity ranges from approximately 3.5 (calcium-aluminium varieties) to 4.3 (iron-rich varieties). Hardness is 6.5 to 7.5 across the group. Garnet is generally durable enough for ring use, with the exception of demantoid and some softer hydrogrossular material.

Garnet has historically not been treated; the natural colour and clarity of garnet do not generally respond to standard heat or irradiation treatments. This makes garnet relatively unusual in the modern coloured stone trade, where treatment is the norm for most species. The recent literature includes reports of heat treatment trials on demantoid for clarity improvement and on spessartine for colour, but these treatments have not become commercial. Buyers can in general purchase garnet with confidence that the colour they see is the natural colour.

Major sources

The principal modern producing regions for gem garnet are the East African Rift system (Kenya, Tanzania, Mozambique, Madagascar) for tsavorite, spessartine, and most colour-change material; Namibia for spessartine and demantoid; the Ural Mountains and Iran for demantoid; Sri Lanka for hessonite, almandine, and pyrope-almandine; Brazil for almandine-pyrope and rhodolite; the Czech Republic for the historical Bohemian garnet trade; and India for almandine. Smaller production occurs in many additional locations.

The market for garnet has grown notably through the 2000s and 2010s, driven by interest in tsavorite as a Colombian emerald alternative, by spessartine in the strong orange colour ranges, and by demantoid as a connoisseur's stone. Prices for the top categories have risen substantially. For tsavorite of clean three-carat-plus material, retail prices reach into the tens of thousands of dollars per carat. Spessartine in fine mandarin colour above five carats reaches several thousand dollars per carat. Demantoid with horse-tail inclusions from the Urals reaches into the high thousands per carat for fine fully-saturated material.

Identification

Garnet identification within the group requires measurement of refractive index, specific gravity, and ideally trace-element chemistry. The refractive index alone is sufficient to distinguish broad compositional ranges, and specific gravity confirms the iron content. Spectroscopic analysis distinguishes chromium- from vanadium-coloured varieties and is essential for confirming demantoid and tsavorite as distinct from the visually similar species peridot, chrome diopside, and emerald.

Inclusion patterns provide additional diagnostic features. The horse-tail inclusions of Russian demantoid, the heat-wave appearance of hessonite, the asbestiform fibres in some uvarovite, and the colour zoning of Malaia garnet are characteristic. Distinguishing pyrope-almandine from spessartine-almandine requires laboratory testing in some cases, particularly where the hue is in the orange-red transition zone.

Historical position

Garnet has the longest documented history of any gem in continuous use. Egyptian use of garnet dates from the predynastic period; Roman and Greek use is extensive; medieval European use is dominated by Bohemian almandine. The Bohemian garnet industry, centred on the Trebnitz region, was the main European source of the deep red almandine-pyrope intermediate material from the seventeenth century through the nineteenth, supplying mass-market garnet jewellery across Europe and the Americas. The discovery of new African and South American sources in the second half of the twentieth century expanded the species count for the trade and reset price expectations for the rare end-members.

For the contemporary trade, garnet is one of the most rewarding species groups for the connoisseur because of the breadth of natural colour, the absence of treatment, and the wide price range from accessible Bohemian material to top tsavorite and demantoid at the highest end.