How to Identify Hornblende (Amphibole): Hardness, Streak, Luster and Tests

The properties that identify Hornblende (Amphibole) — mohs hardness: 5–6 (harder than glass); streak: White to gray; luster: Vitreous to dull luster, opaque; color: Dark green, dark brown, black; cleavage or fracture: Two cleavage directions meeting at about 56 and 124 degrees; uneven fracture; specific gravity: 3.0–3.4, increasing with iron content. Hardness, streak and cleavage are the reliable ones; color alone rarely settles it.

• Cleavage-angle test: amphibole cleavage meets at about 60 and 120 degrees (pyroxene meets at right angles); viewed end-on, cleavage fragments appear diamond- or wedge-shaped.

• Pyroxene: Amphibole cleavage meets at about 60/120 degrees; pyroxene cleavage meets at nearly 90 degrees (right angles).

Common in intermediate to felsic igneous rocks, and in metamorphic rocks such as schist and gneiss; the Morton Gneiss of central Minnesota is a well-known amphibole-bearing rock.

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

‘Hornblende’ was originally an old German mining term. It is no longer used as a distinct mineral name for the most common member of the amphibole group, but in everyday usage, black amphibole crystals are still commonly called hornblende.