How to identify Grossular Garnet

To correctly identify Grossular Garnet (Grossular (Ca3Al2(SiO4)3)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable: colorless, white, yellow, orange, reddish-orange (Hessonite), green (Tsavorite, Hydrogrossular), brown. The specific color can be a key indicator of variety.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to resinous. Tsavorite often has a particularly brilliant vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically forms euhedral to subhedral dodecahedral or trapezohedral crystals. When in matrix, the crystals will be embedded in the host rock, which can be fine-grained to coarse-grained depending on the matrix mineralogy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Most commonly dodecahedral (12-sided) or trapezohedral (24-sided) crystals. Combinations of these forms are also common. Crystals are typically isometric.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Garnets are known for their lack of cleavage, which is a key diagnostic feature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (especially calc-silicate rocks, marbles, skarns), contact metamorphic aureoles around igneous intrusions, serpentinites (for some hydrogrossular). Associated minerals often include diopside, wollastonite, calcite, quartz, epidote, and vesuvianite.

Key Facts

Hardness

6.5-7.5 on Mohs scale

Specific Gravity

3.57-3.73 g/cm³ (can be lower for hydrogrossular, 3.3-3.5 g/cm³)

Crystal System

Isometric (Cubic)

Color

Colorless, white, yellow, orange, reddish-orange, green, brown

Luster

Vitreous to resinous

Transparency

Transparent to opaque

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Calcium aluminum silicate (Ca3Al2(SiO4)3)

Physical characteristics

• Crystal Habit: Typically dodecahedral, trapezohedral, or combinations of these forms; granular, massive.
• Cleavage Type: None (parting may be observed in some cases, but not true cleavage)
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to resinous

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