Black Garnet Identification Guide

What Black Garnet Looks Like

"Black garnet" usually refers to melanite, a titanium-rich black variety of andradite garnet, though very dark almandine can also appear black. It is opaque with a resinous to vitreous, sometimes nearly metallic-looking luster.

• Color: opaque black (melanite) to very dark red-black (almandine in thick pieces)
• Habit: well-formed isometric crystals — rhombic dodecahedra and trapezohedra (12- and 24-faced 'ball-like' crystals)
• Luster: vitreous to resinous, shiny on crystal faces
• Transparency: opaque, though thin slivers of dark almandine may pass deep red light

Step-by-Step Field-ID Checklist

1. Examine crystal shape. Rounded, many-faced isometric crystals (dodecahedra/trapezohedra) strongly indicate garnet.
2. Hardness test. Garnet scratches glass and quartz readily (Mohs 6.5–7.5).
3. Streak test. Streak is white to pale gray/brown despite the black body color.
4. Heft it. Garnet is dense and feels heavy for its size.
5. Backlight thin edges. Almandine may glow deep red; melanite stays black.
6. Check single refraction. Garnet is isometric, so no double images through it.

Key Diagnostic Tests

• Mohs hardness: 6.5–7.5.
• Streak: white to grayish (NOT black) — a useful separator from oxides.
• Cleavage/fracture: no cleavage; uneven to conchoidal fracture, brittle.
• Specific gravity: andradite ~3.8–3.9, almandine ~3.9–4.3 — heavy.
• Magnetism: iron-rich garnets can show a weak magnetic response.
• No acid reaction.

Common Look-Alikes and How to Tell Them Apart

• Black tourmaline (schorl): forms striated prismatic (rod-like) crystals with a rounded-triangular cross-section, not isometric balls; lower density (~3.0–3.2).
• Black spinel: forms octahedra, slightly lower density, and is also singly refractive — distinguish mainly by crystal habit (octahedra vs dodecahedra).
• Magnetite: strongly magnetic, octahedral, with a black streak — garnet's streak is pale.
• Obsidian: glassy, amorphous, conchoidal fracture, no crystal faces, much lower density.
• Black diopside/augite (pyroxene): prismatic with two cleavages near 90°; garnet has no cleavage.

The master clue is crystal habit plus streak: a heavy, many-faced isometric crystal with a pale streak and no cleavage is garnet, while striated rods (tourmaline) or strongly magnetic octahedra (magnetite) are not.

Where Black Garnet Is Found

Melanite andradite occurs in alkaline igneous rocks, skarns, and metamorphosed limestones — notable localities include Italy (Vesuvius area), Germany, France, Mali, and the western USA. Dark almandine forms in mica schists and gneisses worldwide. Star almandine (with asterism) and dark garnets also weather into placer gravels.

Quick Confirmation

A heavy, opaque black crystal with rounded dodecahedral faces, no cleavage, a pale streak, and hardness near 7 is black garnet — most often melanite andradite.