Grandite Garnet Identification Guide

What Grandite Garnet Looks Like

Grandite is not a single species but the solid-solution series between grossular and andradite garnet — hence the blended name. It spans yellow, green, brown, orange, and reddish-brown hues, with the famous green andradite (demantoid) and yellow-green Mali garnets sitting along this series. Luster ranges from vitreous to subadamantine (andradite-rich members can look almost glassy-bright). Crystals are typically transparent to translucent and form the classic garnet shapes: rhombic dodecahedra (12 diamond-shaped faces) and trapezohedra, often as well-rounded, isometric crystals.

Step-by-Step Field-ID Checklist

1. Look for isometric crystal form. Dodecahedral or trapezohedral crystals with no obvious elongation point to garnet.
2. Note the color range. Yellow-green to brown and orange tones are typical of grandite; pure red is more pyrope/almandine.
3. Test hardness. Scratches glass easily (Mohs 6.5–7.5).
4. Check for absence of cleavage. Garnet has no cleavage; it breaks with a conchoidal to uneven fracture.
5. Assess luster. A bright, near-adamantine sparkle suggests an andradite-rich grandite.

Key Diagnostic Tests

• Mohs hardness: 6.5–7.5 (grossular ~6.5–7, andradite ~6.5–7).
• Streak: White.
• Cleavage/fracture: None; conchoidal to uneven fracture.
• Density: ~3.6–3.9, rising toward the andradite end (andradite ~3.8–3.9, grossular ~3.6). Grandite feels heavy for its size.
• No cleavage + isometric habit + high density is the diagnostic combination.
• Refractive index: high, ~1.74–1.89, increasing with andradite content; garnet is singly refractive (isotropic).

Common Look-Alikes and How to Tell Them Apart

• Other garnets (almandine, spessartine, pyrope): Hard to separate by eye; color leans grandite if yellow-green or brown. RI and density measurements pin the species along the series.
• Peridot: Green peridot is doubly refractive (look for doubled back facets) and softer (6.5–7) with distinct cleavage tendencies; grandite is singly refractive.
• Idocrase (vesuvianite): Often found with grossular and looks similar, but vesuvianite forms prismatic (not isometric) crystals and is doubly refractive.
• Green tourmaline / green sapphire: Tourmaline is prismatic with strong pleochroism; sapphire is harder (9). Grandite's isometric habit and lack of pleochroism distinguish it.

Where Grandite Garnet Is Found

Grandite garnets form mainly in contact and regionally metamorphosed limestones (skarns) and in serpentinites. Important sources include Mali (yellow-green grandite), Russia (demantoid in the Urals), Mexico, Italy (Val d'Aosta grossular), Madagascar, and various skarn deposits in the western USA. Search calc-silicate skarns and altered carbonate contacts.