Hornblende
Hornblende is the most common member of the amphibole mineral group, a complex calcium-iron-magnesium aluminosilicate that forms dark, elongated prismatic crystals in a wide variety of igneous and metamorphic rocks. It typically appears black to dark green and is easily recognized by a characteristic cleavage angle of about 56 and 124 degrees. Hornblende is a rock-forming mineral rather than a gem, though well-formed crystals are collected for their sharp form.
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Hornblende properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 |
| Mohs hardness | 5–6 |
| Crystal system | Monoclinic |
| Luster | Vitreous to silky |
| Streak | Pale gray to colorless |
| Cleavage | Perfect prismatic, two directions (~56°/124°) |
| Fracture | Uneven |
| Specific gravity | 3.0–3.4 |
| Transparency | Translucent to opaque |
| Common colors | Black, Dark green, Dark brown |
| Rarity | common |
Geology & background
Hornblende crystallizes from intermediate to mafic magmas and also forms during regional and contact metamorphism, appearing as a major constituent of diorite, andesite, amphibolite, and many gneisses and schists derived from mafic protoliths. Fine, well-terminated crystals suitable for collecting come from localities such as the Kola Peninsula in Russia, Renfrew County in Ontario, and various alpine metamorphic terrains, where hornblende grows into elongated, glossy black prisms embedded in lighter-colored host rock.
Rockhounds encounter hornblende constantly as the dark mineral scattered through granite, diorite, and gneiss, and learning its cleavage angle is one of the most useful field skills for distinguishing it from superficially similar pyroxenes like augite. Because hornblende incorporates water into its crystal structure, it typically forms at somewhat lower temperatures than anhydrous pyroxenes and often rims or replaces pyroxene during late-stage magmatic cooling. Weathered hornblende crystals can alter to chlorite, biotite, or iron oxide over time.
How to identify Hornblende
Hornblende forms elongated, six-sided prismatic crystals, typically black, dark green, or brown, with a vitreous to dull luster and a pale gray to greenish streak. Hardness ranges from 5 to 6, and its two cleavage planes intersect at approximately 56 and 124 degrees, the classic amphibole angle used to separate it from pyroxenes like augite, whose cleavage meets closer to 90 degrees. Crystal habit is long and prismatic rather than the stubby form typical of pyroxene. It is most reliably distinguished from augite by carefully measuring this cleavage angle under magnification.
- 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 Hornblende is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Hornblende 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
- Hornblende's cleavage angles of about 56 and 124 degrees are the standard field test used to separate amphiboles from pyroxenes.
- The name hornblende comes from a German miners' term meaning roughly deceptive ore, since it resembled valuable sulfide minerals but yielded no metal.
- Hornblende contains structural water, unlike the chemically similar pyroxene group, which is entirely anhydrous.
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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