Andradite
Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, one of six species making up the isomorphous garnet group. It occurs in shades of yellow-green, brown, and black depending on trace elements, and includes the gem varieties demantoid, colored green by chromium, and topazolite, a golden-yellow form. Its isometric habit and high refractive index give faceted stones exceptional fire, occasionally surpassing that of diamond.
Andradite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Ca3Fe2(SiO4)3 |
| Mohs hardness | 6.5–7 |
| Crystal system | Cubic |
| Luster | Vitreous to resinous/adamantine |
| Streak | White |
| Cleavage | None |
| Fracture | Conchoidal to uneven |
| Specific gravity | 3.8–3.9 |
| Transparency | Transparent to translucent |
| Common colors | Yellow-green (demantoid), Brown, Black (melanite), Yellow |
| Rarity | common |
Geology & background
Andradite forms mainly through contact and regional metamorphism of impure limestones and calc-silicate rocks, where calcium reacts with iron-bearing fluids to build skarn deposits, and it also crystallizes in serpentinites and certain alkaline igneous rocks. Classic demantoid localities include Russia's Ural Mountains, where crystals grow with fibrous byssolite inclusions forming the distinctive golden horsetail pattern gemologists prize, plus Namibia's Green Dragon mine, a newer source of fine material. Melanite, the black variety colored by titanium, turns up in alkaline igneous rocks such as nepheline syenite.
Collectors seek sharp dodecahedral or trapezohedral crystals pulled from skarn pockets, while gem cutters value clean demantoid for its dispersion. Since andradite has no cleavage, specimens fracture conchoidally rather than along flat planes, a trait that helps separate it from true-cleavage minerals. Rockhounds should note that garnet species are often told apart chemically rather than visually, so color and known locality offer the most practical field clues.
How to identify Andradite
Andradite typically shows a resinous to adamantine, sometimes vitreous luster in colors from yellow-green through brown to black, paired with a white streak despite its dark body color. It has no cleavage, breaking instead with a conchoidal to uneven fracture, and a hardness near 6.5 to 7 resists a steel knife. Crystals commonly form as dodecahedrons or trapezohedrons. It is easily confused with other garnet species such as grossular and, in dark massive form, with tourmaline or epidote.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Andradite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Andradite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Demantoid andradite from the Ural Mountains often contains golden horsetail fibers of byssolite, considered a mark of authenticity rather than a flaw.
- Andradite's dispersion, the property that splits white light into spectral colors, can outdo that of diamond in fine demantoid specimens.
- The black variety melanite was once fashioned into mourning jewelry during the Victorian era.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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