Quick Facts
Formation & Origin
Hornblende crystallizes from intermediate to mafic magmas at temperatures between 700 and 1000 degrees Celsius. It is one of the key minerals in Bowen's reaction series, forming in the continuous reaction series after olivine and pyroxene have crystallized from the cooling melt. Hornblende requires water to form, meaning it only crystallizes from hydrous magmas containing at least 2 to 4 percent dissolved water. This is why it is abundant in continental volcanic arc rocks (like andesites and diorites) where subducting ocean plates release water into the overlying mantle wedge.
The crystal structure consists of double chains of silicon-oxygen tetrahedra running parallel to the c-axis of the crystal, with calcium, magnesium, iron, and aluminum ions occupying octahedral sites between the chains. Hydroxyl groups (OH) sit at the center of each ring of tetrahedra. This double-chain structure controls the crystal's habit (elongated, prismatic) and cleavage (two directions intersecting at 56 and 124 degrees), which are diagnostic features for identification.
In metamorphic rocks, hornblende forms during amphibolite-facies metamorphism at temperatures between 500 and 700 degrees Celsius and pressures of 2 to 10 kilobars. The transition from greenschist facies (dominated by chlorite and actinolite) to amphibolite facies (dominated by hornblende) is one of the most important boundaries in metamorphic petrology. Rocks composed primarily of hornblende and plagioclase feldspar are called amphibolites and represent metamorphosed basalts and gabbros.
Identification Guide
Hornblende forms elongated, prismatic to columnar crystals with a roughly diamond-shaped cross-section. The most diagnostic feature is its cleavage: two sets of cleavage planes intersecting at approximately 56 and 124 degrees, visible on broken cross-sections. This distinguishes it from pyroxene, which has two cleavage planes intersecting at nearly 90 degrees. At hardness 5 to 6, hornblende scratches glass and cannot be scratched with a knife.
Distinguish it from augite (pyroxene), which has a blockier crystal habit and 90-degree cleavage angles. Unlike biotite, hornblende does not split into thin flexible sheets. It differs from tourmaline in having cleavage (tourmaline has no cleavage and shows conchoidal fracture). In hand specimens, hornblende is typically dark green to black with a vitreous luster on fresh surfaces. In thin section under polarized light, it shows strong pleochroism from pale green to dark green or brown, which is a key identifier for petrographers.
Spotting Fakes
Hornblende is a common rock-forming mineral with low commercial value, so deliberate faking is essentially nonexistent. The main identification challenge is distinguishing it from other dark minerals in rock specimens. To confirm hornblende, check the cleavage angle. Break or examine a broken surface and look for the characteristic 56/124-degree cleavage intersection, which distinguishes it from pyroxene's 90-degree cleavage. A hand lens or loupe is helpful. If the specimen is a loose crystal, examine the cross-section shape. Hornblende crystals are typically six-sided (pseudo-hexagonal) with a diamond-like profile. If someone sells a dark mineral as 'black tourmaline' but it shows two cleavage directions, it may actually be hornblende.
Cultural & Metaphysical Traditions
Presented as cultural traditions, not scientific evidence
Hornblende has a limited but growing presence in contemporary crystal practice. It is sometimes used as a grounding stone, with practitioners valuing its deep connection to the earth's crust and mantle processes. In some modern traditions, hornblende is associated with composure, patience, and endurance, reflecting its role as a stable mineral that persists through intense geological conditions. The name 'hornblende' comes from the German words 'horn' (referring to its dark color) and 'blenden' (to deceive or dazzle), because miners found it looked like a valuable ore but contained no useful metal. This history has led some traditions to associate it with discernment and seeing through illusion.
Where It's Found
Large prismatic crystals in metamorphic terranes of the Canadian Shield
Classic locality for well-formed hornblende crystals in gneiss
Fine euhedral crystals in Alpine metamorphic rocks
Abundant in diorites and amphibolites
Large crystals in contact metamorphic zones
Price Guide
Good to Know
Scratch test: At hardness 5.5, Hornblende resists scratching from a knife but can be scratched by quartz. Best for pendants and earrings rather than rings.
Sources: Found in 5 notable locations worldwide, from Canada to Russia.
Heft test: Hornblende has average mineral density (3.0–3.5). It feels about as heavy as you'd expect from a stone its size.
Related Minerals
Similar dark mineral but a pyroxene with 90-degree cleavage
Related amphibole, greener color, forms at lower metamorphic grades
Another common dark rock-forming mineral but with sheet cleavage