How to identify Black Granite

To correctly identify Black Granite (Gabbro or Diorite (commercial granite)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically very dark gray to black. Some varieties may show a greenish or bluish tint due to specific mineral compositions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous, depending on the mineral constituents. Plagioclase feldspar often has a pearly luster on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained) to medium-grained, with interlocking crystals visible to the naked eye. Some varieties (e.g., diabase) can be ophitic or subophitic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Anhedral to subhedral crystals, forming an interlocking mosaic. Individual mineral grains (e.g., pyroxene, plagioclase) may show characteristic crystal habits if well-formed.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Minerals within the rock (e.g., pyroxene, amphibole, plagioclase) exhibit distinct cleavage planes. Pyroxenes typically have two cleavages at nearly 90 degrees, amphiboles at approximately 56 and 124 degrees, and plagioclase has two cleavages at nearly 90 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in large intrusive bodies (plutons, batholiths, sills, dikes) within continental and oceanic crust, often associated with magmatic arcs, rift zones, and large igneous provinces.

Key Facts

Hardness

6-7 on the Mohs scale (due to constituent minerals like feldspar, pyroxene, amphibole).

Specific Gravity

2.8-3.3 g/cm³ (varies with mineral composition; gabbro is typically denser than diorite).

Crystal System

Monoclinic (pyroxene, amphibole), Triclinic (plagioclase feldspar). The rock itself is an aggregate of crystals.

Color

Dark gray to black.

Luster

Vitreous to sub-vitreous.

Transparency

Opaque (as a rock aggregate). Individual mineral grains can be translucent to opaque.

Fracture

Irregular to conchoidal (individual mineral grains). The rock typically exhibits a blocky fracture.

Cleavage

Present in constituent minerals (e.g., pyroxene, amphibole, plagioclase).

Composition

Gabbro: Predominantly calcic plagioclase feldspar (>50%) and pyroxene (clinopyroxene, orthopyroxene), with minor olivine, amphibole, and iron-titanium oxides. Diorite: Predominantly plagioclase feldspar (andesine to oligoclase) and amphibole (hornblende), with lesser biotite, pyroxene, and quartz (up to 5%). Anorthosite: Composed almost entirely of plagioclase feldspar (>90%).

Physical characteristics

• Crystal Habit: Granular, interlocking anhedral to subhedral crystals.
• Cleavage Type: Good to perfect in constituent minerals (e.g., two directions in pyroxene and amphibole, two in plagioclase).
• Fracture Type: Irregular to blocky for the rock mass; conchoidal to uneven for individual mineral grains.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous.

Explore Black Granite