Augite
Augite is a common rock-forming pyroxene, a calcium-magnesium-iron silicate with the general formula (Ca,Mg,Fe)2Si2O6, and one of the most abundant dark minerals in basalt and gabbro. It crystallizes in the monoclinic system and typically appears as stubby black to greenish-black prisms with a characteristic near-right-angle cleavage. Augite is a rock-forming mineral rather than a gem material, valued mainly by collectors for its well-formed crystals.
Augite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 |
| Mohs hardness | 5.5–6 |
| Crystal system | Monoclinic |
| Luster | Vitreous to resinous |
| Streak | Grayish green to colorless |
| Cleavage | Good prismatic (~87°/93°) |
| Fracture | Uneven |
| Specific gravity | 3.2–3.6 |
| Transparency | Translucent to opaque |
| Common colors | Black, Dark green, Brown |
| Rarity | common |
Geology & background
Augite crystallizes directly from cooling mafic and ultramafic magma, forming one of the first minerals to solidify alongside plagioclase feldspar and olivine in basalt, gabbro, and diabase. Its short, stubby eight-sided crystals turn up frequently in volcanic rock from Vesuvius and Etna, and large individual crystals have been collected from lava flows in Germany's Eifel district. Augite also occurs in some metamorphic rocks derived from mafic igneous protoliths.
Collectors often confuse augite with hornblende, since both are dark, prismatic, rock-forming silicates found in similar rock types, but augite's cleavage angles of roughly 87 and 93 degrees distinguish it from amphibole's 56 and 124 degree cleavage. Weathering slowly alters augite into chlorite or serpentine minerals, so fresh, lustrous crystal faces make the best specimens. Because augite rarely forms large, gem-quality crystals, its collector value rests mainly on crystal form and rock context rather than facetable clarity.
How to identify Augite
Augite forms short, stubby prismatic crystals with a nearly square cross-section, typically black, dark green, or brown, with a vitreous to dull luster and a colorless to pale gray-green streak. Hardness falls between 5.5 and 6, and its two cleavage directions meet at angles close to 87 and 93 degrees, a key diagnostic separating it from the amphibole group. It occurs embedded in dark volcanic groundmass far more often than as loose euhedral crystals, and beginners frequently mistake it for hornblende until the cleavage angle is checked.
- 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 Augite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Augite 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
- Augite's cleavage planes intersect at roughly right angles, close to 87 and 93 degrees, which is the classic pyroxene diagnostic.
- Large lustrous augite crystals up to several centimeters have been collected from volcanic ejecta in Germany's Eifel volcanic field.
- Augite is one of the first minerals to crystallize from basaltic magma, alongside olivine and calcium-rich plagioclase.
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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