Identifying Black Minerals and Rocks

Augite

Augite is a standard black or brownish-black pyroxene mineral of dark igneous rocks and some high-grade metamorphic rocks. Its crystals and cleavage fragments are nearly rectangular in cross-section (at angles of 87 and 93 degrees). These are the main things that distinguish it from hornblende (see below).

Characteristics: Glassy luster; hardness of 5 to 6.

Biotite

Biotite forms shiny, flexible flakes that are typically deep black or brownish-black in color. These flakes can occur as large sheet-like "book crystals" in pegmatites, which are coarse-grained igneous rocks. Biotite is also widespread in other igneous and metamorphic rocks. Additionally, tiny detrital flakes of biotite may be found in dark sandstones, which are sedimentary rocks. 

Characteristics: Glassy to pearly luster; hardness of 2.5 to 3.

Chromite

Chromite is a chromium-iron oxide found in pods or veins in bodies of peridotite and serpentinite—just look for brown streaks. It may also be segregated in thin layers near the bottom of large plutons, or former bodies of magma, and is sometimes found in meteorites. It may resemble magnetite but rarely forms crystals and is only weakly magnetic.

Characteristics: Submetallic luster; hardness of 5.5.

Hematite

Hematite, an iron oxide, is the most common black or brownish-black mineral in sedimentary and low-grade metasedimentary rocks. It varies greatly in form and appearance, but all hematite produces a reddish streak.

Characteristics: Dull to semimetallic luster; hardness of 1 to 6.

Hornblende

Hornblende is the typical amphibole mineral in igneous and metamorphic rocks. Look for glossy black or dark green crystals and cleavage fragments forming flattened prisms in cross-section (corner angles of 56 and 124 degrees). Crystals may be short or long, and even needle-like in amphibolite schists.

Characteristics: Glassy luster; hardness of 5 to 6.

Ilmenite

Crystals of this titanium-oxide mineral are sprinkled in many igneous and metamorphic rocks, but they're sizeable only in pegmatites. Ilmenite is weakly magnetic and produces a black or brownish streak. Its color can range from dark brown to red.

Characteristics: Submetallic luster; hardness of 5 to 6.

Magnetite

Magnetite (or lodestone) is a common accessory mineral in coarse-grained igneous rocks and metamorphic rocks. It may be gray-black or have a rusty coating. Crystals are common, with striated faces shaped like octahedrons or dodecahedrons. Look for a black streak and a strong attraction to a magnet. 

Characteristics: Metallic luster; hardness of 6.

Pyrolusite, Manganite, and Psilomelane

These manganese-oxide minerals usually form massive ore beds or veins. The mineral-forming black dendrites between sandstone beds are generally pyrolusite. Crusts and lumps are typically called psilomelane. In all cases, the streak is sooty black. These minerals release chlorine gas when exposed to hydrochloric acid.

Characteristics: Metallic to dull luster; hardness of 2 to 6.

Rutile

The titanium-oxide mineral rutile usually forms long, striated prisms or flat plates, as well as golden or reddish whiskers inside rutilated quartz. Its crystals are widespread in coarse-grained igneous and metamorphic rocks. Its streak is light brown.

Characteristics: Metallic to adamantine luster; hardness of 6 to 6.5.

Stilpnomelane

This uncommon glittering black mineral, related to the micas, is found primarily in high-pressure metamorphic rocks with high iron content such as blueschist or greenschist. Unlike biotite, its flakes are brittle rather than flexible.

Characteristics: Glassy to pearly luster; hardness of 3 to 4.

Tourmaline

Tourmaline is common in pegmatites. It's also found in coarse-grained granitic rocks and some high-grade schists. It typically forms prism-shaped crystals with a cross-section shaped like a triangle with bulging sides. Unlike augite or hornblende, tourmaline has poor cleavage and is also harder than those minerals. Clear and colored tourmaline is a gemstone. The typical black form is sometimes called schorl.

Characteristics: Glassy luster; hardness of 7 to 7.5.

Other Black Minerals

Uncommon black minerals include allanite, babingtonite, columbite or tantalite, neptunite, uraninite, and wolframite. Many other minerals may occasionally take on a black appearance, whether they are ordinarily green (chlorite, serpentine), brown (cassiterite, corundum, goethite, sphalerite), or other colors (diamond, fluorite, garnet, plagioclase, spinel).