It tends to form slender prismatic to bladed crystals, diamond-shaped in cross section, or is present as irregular grains or fibrous masses.

[10] It is highly variable in composition, and includes at least five solid solution series: In addition, titanium, manganese, or chromium can substitute for some of the cations and oxygen, fluorine, or chlorine for some of the hydroxide (OH).

[12] Hornblende is a common constituent of many igneous and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss, and schist.

[14] A rare variety of hornblende contains less than 5% of iron oxide, is gray to white in color, and is named edenite from its locality in Edenville, Orange County, New York.

[15][16] The word hornblende is derived from German Horn ('horn') and blende ('deceive'), in allusion to its similar appearance to metal-bearing ore minerals.