People trying to identify hornblende in hand sample often find it helpful to look for “sparkly tree bark.”

Physical Properties
Chemical formula (K,Na)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
Class Inosilicate
Double chains of linked tetrahedra
Crystal system Monoclinic
Habit Massive
Color Black or dark green
Hardness 5 to 6
Specific gravity 3.0 to 3.5
Cleavage Two perfect prismatic {110}
Fracture Uneven
Luster Vitreous
Transparency Translucent
Streak White
Optical Properties
PPL Brown, green, blue-green, or yellow-brown
Moderate to strong pleochroism
“60/120” cleavage (actually 54° and 124°)
XPL 2nd order oranges; can range up to 4th order
δ 0.02
Twinning Contact twins are common
after Perkins, 332-333

Hornblende in Hand Sample

A different hornblende
Hornblende in amphibolite
Hornblende in hornblende muscovite schist
Black hornblende and pinkish-red corundum (ruby) in green zoisite
Scanning electron micrograph of hornblende
Scanning electron micrograph of a hornblende grain plucked from a garnet-rich sand from the Moray Coast in northeastern Scotland (there’s diatom tests on it!)

Hornblende in Thin Section

Thin Section GigaPans

Hornblende in augen gneiss, plane polars
Hornblende in augen gneiss, crossed polars

Hornblende, PPL

Hornblende, XPL

Further Reading

Hornblende at webmineral.com
Hornblende at mindat.org