Gemology Fast Track / Unit 13 of 24 · Major species · 60 min
The garnet group
Goal: Separate garnet species by RI/SG/spectrum and recognise valuable varieties.
All garnets: cubic, singly refractive, no pleochroism, often weak ADR. RI and SG identify the species.
PyropeRI 1.714–1.742 · SG 3.62–3.87Dark red; Cr lines in chrome pyrope.
RhodoliteRI 1.745–1.770 · SG 3.74–3.94Pyrope-almandine; raspberry/purple.
AlmandineRI 1.775–1.830 · SG 3.93–4.30505/527/576 nm bands; rutile needles.
SpessartineRI 1.780–1.815 · SG 4.12–4.20Orange; Mn bands 412/424/432 nm; wavy inclusions.
Grossular / tsavoriteRI 1.730–1.760 · SG 3.57–3.73Tsavorite V/Cr green; hessonite 'treacle'.
Andradite / demantoidRI 1.88 (OTL) · SG 3.81–3.87Dispersion 0.057; horsetail inclusions.
UvaroviteRI ~1.74–1.87 · SG ~3.71–3.81Druzy green; too small to facet.
High-value garnets
Demantoid
Top: Russian with horsetails. OTL + SR + huge fire.
Tsavorite
Vivid green, rarely >5 ct. Red under CF.
Mandarin spessartine
Pure orange, Namibia/Nigeria.
Colour-change garnet
Blue-green → purple (Bekily). SR separates from alexandrite.
Quick separation
Garnet vs rubySR vs DR; garnet inert in UV; almandine 3-band spectrum.
Tsavorite vs emeraldSR 1.74 vs DR 1.57–1.60; tsavorite brighter, cleaner.
Hessonite vs spessartineRI 1.74 vs 1.79–1.81; treacle texture vs wavy fluid.
Garnet-topped doubletRed ring on white paper; lustre change on crown.
Gems in this unit (22)
Practical
Garnets are cheap to buy: get 3–5 assorted (rhodolite, almandine, hessonite, spessartine) and measure SG. The SG spread alone (3.6 → 4.2) teaches the group.
Unit quiz
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