How to Identify Hornblende: Properties & Lookalikes

Dark hornblende amphibole hand specimen about four centimeters across from near Kragerø, Norway — Hornblende amphibole specimenPhoto: Shannon Heinle · CC0 1.0 · Wikimedia Commons

Classification

What is hornblende?

Hornblende refers to compositionally complex calcic amphiboles containing varying calcium, sodium, magnesium, iron, aluminum, silicon, and hydroxyl. Precise amphibole naming can require chemical and crystallographic analysis.

Appearance

What does hornblende look like?

Crystals are commonly long, bladed, columnar, or fibrous-looking, with dark green to black color and glassy to dull luster. Cross sections may appear diamond-shaped or roughly six-sided.

Cleavage surfaces intersect at oblique angles and can create splintery fragments. Fine grains in dark rock may be difficult to separate visually from pyroxene, biotite, or tourmaline.

Field tests

Identification tests

Begin with observation and use the least destructive test first. Results can change with weathering, mixtures, coatings, and grain size. Review the step-by-step field-test methods before altering an unfamiliar specimen.

Hardness

About 5 to 6

Harder than a knife blade in many directions but generally softer than quartz.

Cleavage

Two good directions near 56 and 124 degrees

The oblique amphibole cleavage is the strongest distinction from pyroxene.

Habit

Elongate prisms or splintery aggregates

Supports amphibole rather than blockier augite.

Streak

White to pale gray

Usually not diagnostic.

Useful procedures: Mohs hardness test · Cleavage versus fracture · Mineral streak test

Comparison

Common lookalikes

Augite

Usually blockier with two cleavage directions closer to 90 degrees.

Tourmaline

Harder, strongly lengthwise-striated, commonly triangular in cross section, and lacks hornblende’s good cleavage.

Biotite

Splits into flexible thin sheets along one perfect basal cleavage.

Geologic origin

How does it form?

Hornblende crystallizes in many intermediate to felsic igneous rocks and forms during regional or contact metamorphism where water is available.

It is common in diorite, granodiorite, amphibolite, gneiss, schist, and related rocks, but a dark grain alone does not establish the host-rock name.

Where it occurs

Geologic setting and occurrence

Hornblende is widespread in continental igneous and metamorphic rocks and may occur as individual crystals, aligned metamorphic grains, or a major component of amphibolite.

Visual identification generally supports “hornblende-like amphibole”; precise species classification may require laboratory chemistry.

Human use

Uses and significance

Hornblende has limited direct industrial use but is important for classifying rocks, reconstructing metamorphic conditions, and interpreting magma water content and evolution.

Safety and limits

Field notes

Measure cleavage angles across several grains rather than judging one break.
Do not call every black prismatic mineral hornblende.
Avoid producing dust from fibrous or altered amphibole-bearing material.

This guide supports preliminary field identification, not a laboratory determination, appraisal, hazard assessment, or statement of ownership or collecting permission. Verify land status and local rules before handling or collecting material.

Source basis

Authoritative sources

LithoMap wrote this guide in original language using the scientific concepts and physical-property standards supported by these sources.

Minerals and their physical properties U.S. National Park Service
Mohs Hardness Scale U.S. National Park Service
Natural Gemstones: Mineral Gemstones U.S. Geological Survey

Editorial review: September 17, 2026.

Back to mineral identification