What garnet actually is
Garnet is a supergroup of more than 30 silicate minerals sharing the common formula X₃Y₂(SiO₄)₃ and isometric cubic crystal symmetry, but spanning a wide chemical compositional range across two principal end-member series[IMA][GIA].
The pyralspite series occupies the iron-magnesium-manganese half: pyrope at the magnesium end, almandine at the iron end, and spessartine at the manganese end. The ugrandite series occupies the calcium half: uvarovite, grossular, and andradite[USGS][Gem-A].
The commercial gem garnets are mostly intermediate solid solutions across these end-members rather than pure end-member compositions, which is why the trade uses variety names like rhodolite for a pyrope-almandine intermediate, Malaya for a pyrope-spessartine intermediate, and tsavorite for a chromium-and-vanadium-bearing grossular[IGS].
The optical fingerprints labs use to identify garnet vary across the supergroup. Refractive index ranges from 1.72 in tsavorite to 1.94 in andradite, specific gravity from 3.5 in tsavorite to 4.3 in almandine, dispersion is unusually high in the andradite-derived demantoid variety at 0.057, exceeding diamond's 0.044[IGS][GIA]. Mohs hardness ranges 6.5–7.5, generally putting demantoid and tsavorite at the slightly softer end and almandine at the harder end. Cleavage is absent across the group, which contributes to durability despite the moderate hardness.
Variety chemistry
The garnet supergroup's defining characteristic is that the gemological character, color, dispersion, hardness, durability, is determined by which end-members dominate the solid-solution composition[GIA][Gübelin].
Almandine-Pyrope (the "red garnet" union) covers the iron-magnesium pyralspite intermediate, purplish red to dark red with the trade transitioning to "rhodolite" for the more purplish material. Almandine end-members are higher in iron and slightly darker. Pyrope end-members are higher in magnesium and slightly more orange[IGS].
Spessartite is manganese-aluminum garnet, the bright orange to reddish orange of the "Mandarin" trade-name material from Namibia and Mozambique[GRS]. Spessartine's manganese chromophore produces the saturated orange that no other red-orange gemstone matches.
Demantoid is the andradite green variety. Its calcium-iron framework hosts chromium impurity at parts-per-thousand levels, producing the green color, while iron drives the unusually high dispersion of 0.057[Lotus]. Demantoid was first described from the Russian Ural Mountains in the late 19th century, and Russian material remains the historic premium reference. Namibian, Iranian, and Madagascar production has grown the supply.
Tsavorite is grossular green. Chromium and vanadium impurities in the calcium-aluminum framework produce a vivid emerald-comparable green, but without the included character of emerald[GRS]. Tsavorite was discovered in the Tsavo region of Kenya in 1967 and named by Henry Platt of Tiffany and Co. Tanzanian material from the same metamorphic belt completes the modern supply.
Hessonite is the grossular orange-to-brown variety, sometimes called "cinnamon stone", with a characteristic "treacly" inclusion pattern from minute metamorphic-mineral particles[GIA]. East African and Sri Lankan deposits dominate.
Malaya is a pyrope-spessartine intermediate, peach to pinkish orange with rare color-change behavior in select Tanzanian and Madagascar material driven by both vanadium and chromium chromophores[IGS].
Origin science
Geographic origin determination for fine garnet varies by variety. Tsavorite, demantoid, and Mandarin spessartite have established origin reports from SSEF, GRS, GIA, and Gübelin, leveraging trace-element chemistry and inclusion microscopy[SSEF][Gübelin]. Russian demantoid is distinguished from Namibian by inclusion patterns. Russian material carries the diagnostic horsetail asbestiform-chrysotile fibers that Namibian rarely shows, despite both being andradite chemistry[Lotus].
Geological context across varieties: tsavorite forms in metamorphic graphitic gneiss, demantoid in serpentinites and skarns, Malaya garnet in placer deposits derived from East African metamorphic terrains, almandine and pyrope across pelitic schists and amphibolites worldwide[USGS].
Treatment landscape
Garnet commerce is dominated by untreated material. The supergroup is gemologically among the least-treated gem species categories, and most varieties enter the trade in their natural state[GIA][LMHC]. Demantoid receives occasional low-temperature heat treatment to reduce brown hues per GIA, but the practice is undetectable and uncommon in commerce. Most pyrope-almandine red garnet is sold without enhancement. Spessartite, tsavorite, hessonite, and Malaya garnet are essentially never treated[CIBJO].
Synthetic garnets, yttrium aluminum garnet (YAG) and gadolinium gallium garnet (GGG), exist as industrial materials and as diamond simulants but rarely enter the natural-garnet supply chain[IGS]. The price differential between natural and synthetic remains wide enough that incentive to misrepresent is low.
Phenomena science
Demantoid horsetail inclusions are radiating bundles of asbestiform chrysotile fibers from the metamorphic host rock, captured during garnet crystal growth[Gem-A][Lotus]. These inclusions are diagnostic for demantoid garnet from the Russian Ural Mountains and represent a price-positive collector signature unique among gem species. Most species penalize visible inclusions, but the horsetail in demantoid is sought by collectors precisely because the inclusion pattern is itself a provenance signature.
Color change in select Malaya garnet from pyrope-spessartine intermediate compositions with chromium and vanadium chromophores produces a daylight-to-incandescent hue shift[IGS][GIA]. The phenomenon is rare and the commercial market is narrow.
Star and cat's-eye garnet from almandine-pyrope and rhodolite material, requiring oriented inclusion sets and cabochon cutting, are documented but very rare across the supergroup[GIA].
Species identity synthesized from the authoritative sources cited below.