Grossular Garnet Identification Guide

What Grossular Garnet Looks Like

Grossular is the calcium-aluminum member of the garnet group (Ca₃Al₂(SiO₄)₃). It is the most color-diverse garnet: vivid green (tsavorite), pale mint, golden, honey, orange-brown cinnamon (hessonite), and even near-colorless. Crystals are typically well-formed rhombic dodecahedra or trapezohedra with a glassy, vitreous to resinous luster. Hessonite often shows a characteristic swirled, "heat-wave" or treacle internal texture.

• Color: green, mint, yellow, honey, cinnamon-brown, orange, colorless
• Transparency: transparent to translucent
• Luster: vitreous to resinous
• Habit: isometric dodecahedra/trapezohedra; granular masses in metamorphic rock

Step-by-Step Field-ID Checklist

1. Look at crystal shape. Equant 12- or 24-sided crystals with no cleavage strongly suggest garnet.
2. Test for no cleavage. Garnets fracture conchoidally and show no flat cleavage planes.
3. Check hardness. It scratches glass and quartz readily.
4. Weigh it. Garnets feel noticeably heavy for their size.
5. Inspect color and matrix. Green or cinnamon stones in calc-silicate or skarn rock favor grossular; look for the roiled inclusions of hessonite.

Key Diagnostic Tests

• Mohs hardness: 6.5–7.5 — scratches glass and quartz.
• Streak: white.
• Cleavage: none; uneven to conchoidal fracture.
• Specific gravity: ~3.4–3.7 — distinctly heavy.
• Crystal system: isometric (cubic) — single, undivided refractive index, so it stays dark/inert under a polariscope (isotropic).
• Acid: no reaction.

Common Look-Alikes and How to Tell Them Apart

• Tsavorite (green grossular) vs. emerald: emerald is hexagonal, much softer-feeling under wear, has lower density, and shows different inclusions; grossular has no cleavage and higher SG.
• Hessonite vs. spessartine garnet: spessartine is generally more pure orange; hessonite shows its signature treacly, swirled inclusions and is the Ca-Al type.
• Andradite (demantoid) garnet: also green but has higher dispersion (fire) and higher SG; chemistry differs (Ca-Fe).
• Peridot: green but only 6.5–7 hardness, strongly doubly refractive (doubled back facets), and has cleavage tendencies.
• Citrine/yellow quartz: lower SG, hexagonal, and softer than garnet.

The core test set is isometric crystals + no cleavage + high SG (3.4–3.7) + hardness near 7, which separates grossular from beryl, peridot, and quartz.

Where Grossular Garnet Is Found

Grossular forms mainly in contact and regionally metamorphosed impure limestones and skarns (calc-silicate rocks). Famous sources include Kenya and Tanzania (tsavorite), Sri Lanka (hessonite), Mexico, Canada (Jeffrey Mine, Quebec), Mali, Russia, and Italy's classic localities.

Quick Field Summary

A heavy, glassy, well-formed dodecahedral crystal with no cleavage that scratches glass and quartz — in green, mint, honey, or cinnamon — hosted in calc-silicate or skarn rock is grossular garnet, not emerald, peridot, or quartz.