Andradite Garnet Identification Guide
How to identify andradite garnet, a calcium-iron garnet including demantoid and melanite, by its dodecahedral form, high dispersion, and density.
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What Andradite Garnet Looks Like
Andradite is the calcium-iron garnet, Ca3Fe2(SiO4)3, crystallizing in the isometric (cubic) system. It occurs in several varieties: demantoid (vivid green, the most prized), melanite (opaque black), and topazolite (yellow to honey). Color thus ranges from green and yellow-brown to black. Luster is vitreous to subadamantine (resinous), and andradite has notably high dispersion — even higher than diamond — giving green demantoid its strong fire. Crystals are commonly dodecahedra (12 faces) or trapezohedra (24 faces), well-formed and equant.
Step-by-Step Field-ID Checklist
- Look for isometric crystal form — rounded dodecahedra/trapezohedra with no preferred direction.
- Confirm no cleavage — garnets break with uneven-to-conchoidal fracture.
- Scratch test — Mohs 6.5–7; scratches glass.
- Heft it — garnets feel heavy for their size (high density).
- For green stones, look for fire (dispersion) and the characteristic 'horsetail' inclusions of demantoid under a loupe.
Key Diagnostic Tests
- Mohs hardness: 6.5–7 (slightly softer than pyrope-almandine garnets).
- Streak: white.
- Cleavage: none; uneven to conchoidal fracture.
- Specific gravity: ~3.8–3.9 — the densest of the common garnets.
- Refractive index: very high (~1.85–1.89) with strong dispersion.
- Inert to acid; melanite/iron-rich types may be weakly magnetic.
Common Look-Alikes
- Other garnets (pyrope, almandine, grossular): all isometric and cleavage-free; andradite is separated by its higher RI, higher dispersion, and higher density (SG ~3.85). Demantoid's 'horsetail' chrysotile inclusions are nearly unique.
- Peridot / chrome diopside (green): peridot is doubly refractive with strong birefringence and lower density; diopside has cleavage (garnet has none).
- Emerald / tsavorite (green grossular): emerald is hexagonal with cleavage tendencies; tsavorite is also garnet but lower SG and RI than demantoid.
- Melanite vs black tourmaline/spinel: tourmaline is trigonal with striated prisms; spinel forms octahedra; andradite forms dodecahedra and is denser than tourmaline.
Where It Is Found
Andradite forms in contact and regional metamorphism of impure limestones (skarns) and in serpentinites and alkaline igneous rocks. Demantoid comes famously from the Ural Mountains (Russia), Namibia, Madagascar, and Iran; melanite from alkaline volcanics in Italy and elsewhere; topazolite from the Italian Alps. Skarns and serpentine bodies worldwide host the species.
Frequently asked questions
How can you tell if it's andradite garnet?
Look for an isometric garnet (dodecahedral crystals, no cleavage, conchoidal fracture, Mohs 6.5–7) that is unusually dense (SG ~3.85) with high luster and strong dispersion. Green demantoid often shows distinctive horsetail inclusions.
What does andradite garnet look like?
It appears as well-formed 12- or 24-faced equant crystals or gems in green (demantoid), yellow-brown (topazolite), or black (melanite), with a glassy-to-resinous luster and, in green stones, noticeable fire.
What is demantoid garnet?
Demantoid is the vivid green, iron-and-chromium-bearing variety of andradite. It is the most valuable garnet, prized for its intense color and exceptional dispersion (fire), and often identified by horsetail-shaped inclusions.
Andradite vs almandine garnet — how do they differ?
Both are isometric garnets, but andradite is the calcium-iron type (green/yellow/black), denser and more dispersive, while almandine is the iron-aluminum type (deep red), slightly harder. Color, density, and dispersion separate them.