The garnet group
The garnet group comprises a series of related silicate minerals sharing a common cubic crystal structure. The principal gem-grade species are:
- Almandine (Fe₃Al₂(SiO₄)₃) — the most common species; colors range from deep red to brownish red.
- Pyrope (Mg₃Al₂(SiO₄)₃) — red to purplish red.
- Spessartine (Mn₃Al₂(SiO₄)₃) — orange to red-orange.
- Grossular (Ca₃Al₂(SiO₄)₃) — yellow, green (tsavorite), pink, colorless.
- Andradite (Ca₃Fe₂(SiO₄)₃) — yellow, green (demantoid), brown.
Mohs hardness across the group is 6.5 to 7.5; specific gravity is 3.5 to 4.3 depending on species.
Notable varietal terms
- Tsavorite — chromium-bearing green grossular, first identified at Tsavo, Kenya, in the 1960s.
- Demantoid — chromium-bearing green andradite from the Russian Urals (historic source) and Madagascar.
- Rhodolite — pyrope-almandine intermediate composition with rose to purplish-red color.
- Mali garnet — grossular-andradite intermediate, yellow-green, from Mali.
- Color-change garnet — pyrope-spessartine intermediate showing daylight-to-incandescent color shift.
Sources
- Tanzania — Merelani and Umba valley: tsavorite, spessartine, color-change.
- Kenya — Tsavo and Lualenyi: tsavorite (the type locality).
- Mozambique — pyrope-almandine, color-change, rhodolite.
- Madagascar — demantoid, mali, color-change.
- Russia — Ural Mountains: historic source of demantoid.
- Brazil, Namibia, India — almandine, pyrope, varietals.
Treatments
Garnets are typically not treated. Heat treatment is occasionally applied to spessartine and rhodolite but is uncommon and is generally disclosed when performed. Synthetic garnet (yttrium aluminum garnet, gadolinium gallium garnet) is commercially produced for laser and gem-simulant applications and is identifiable in the laboratory.
Lapidary considerations
The garnet group is well-suited to lapidary work: the hardness range supports standard diamond grit progressions, and most species polish cleanly with cerium oxide or fine diamond paste. Demantoid and tsavorite show significant dispersion and benefit from cuts that emphasize fire.
Garnets are isotropic (singly refractive) — orientation during preform layout does not affect color, simplifying species-specific cutting workflows.