Black Granite

Description

The commercial term 'Black Granite' is a misnomer in geological terms. True granite is a felsic igneous rock, rich in quartz and feldspar, and typically light-colored. What is commercially sold as 'Black Granite' is almost always a mafic igneous rock, such as gabbro, norite, diabase (dolerite), anorthosite, or sometimes a very dark diorite or even basalt. These rocks are characterized by a high content of mafic minerals (rich in magnesium and iron), such as pyroxene, amphibole, and olivine, along with plagioclase feldspar. They are typically dark gray to black in color, dense, and very hard. The texture is usually phaneritic (coarse-grained) for plutonic varieties like gabbro and diorite, or aphanitic (fine-grained) to porphyritic for volcanic varieties like basalt, or ophitic/subophitic for diabase.

How to Identify

Color

Typically dark gray to black. May exhibit a 'salt and pepper' appearance with white plagioclase and black mafic minerals, or be uniformly dark.

Luster

Vitreous to sub-vitreous on fresh surfaces, especially for feldspar and pyroxene crystals. Polished surfaces are highly reflective.

Texture

Phaneritic (coarse-grained) for gabbro and diorite, with interlocking crystals visible to the naked eye. Diabase often has an ophitic or subophitic texture. Basalt is aphanitic (fine-grained) or porphyritic.

Crystal Form

Individual mineral crystals (plagioclase, pyroxene, amphibole, olivine) are typically anhedral to subhedral, interlocking to form a massive rock.

Cleavage

Minerals within the rock (e.g., pyroxene, amphibole, plagioclase) exhibit characteristic cleavage planes (e.g., pyroxene: two cleavages at ~90 degrees; amphibole: two cleavages at ~56 and ~124 degrees; plagioclase: two cleavages at ~90 degrees). The rock itself does not have a single cleavage plane.

Geological Environment

Intrusive mafic rocks (gabbro, diorite) form in large plutons, batholiths, or layered intrusions deep within continental or oceanic crust. Diabase forms in dikes and sills. Basalt forms from volcanic eruptions on the Earth's surface or seafloor.

Key Facts

• Hardness: 6-7 on Mohs scale (due to constituent minerals like feldspar and pyroxene)
• Specific Gravity: 2.8-3.3 g/cm³ (denser than true granite)
• Crystal System: Monoclinic (pyroxene, amphibole), Triclinic (plagioclase)
• Color: Dark gray to black
• Luster: Vitreous to sub-vitreous
• Transparency: Opaque (in hand specimen)
• Fracture: Uneven to conchoidal (for individual minerals), generally irregular for the rock mass
• Cleavage: Present in constituent minerals (e.g., pyroxene, amphibole, plagioclase), but not a rock cleavage
• Composition: Predominantly plagioclase feldspar (labradorite to bytownite) and mafic minerals such as pyroxene (augite), amphibole (hornblende), and sometimes olivine. Quartz is typically absent or present in very minor amounts (<5%).

Quick Check

• Color: Dark gray to black
• Luster: Vitreous to sub-vitreous (on fresh surfaces), highly reflective (when polished)
• Streak: White to very light gray (due to plagioclase, though mafic minerals are dark, their streak is often light or absent on unglazed porcelain)

Physical Characteristics

• Crystal Habit: Granular, interlocking anhedral to subhedral crystals (phaneritic).
• Cleavage Type: Good to perfect in constituent minerals (e.g., two directions at ~90° in pyroxene, two directions at ~56°/124° in amphibole, two directions at ~90° in plagioclase).
• Fracture Type: Irregular to sub-conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous.

Formation

Formed from the slow cooling and crystallization of mafic magma deep within the Earth's crust (for gabbro, diorite) or as intrusive dikes/sills (diabase/dolerite). Some commercial 'black granites' are extrusive basalts.

Usage

Primarily used as a dimension stone for countertops, flooring, wall cladding, monuments, and grave markers due to its durability, aesthetic appeal, and ability to take a high polish. Also used as crushed stone for construction aggregate.

Age Distribution

Precambrian to Cenozoic, depending on the specific mafic rock type.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock (Plutonic or Hypabyssal)

Group

Mafic Rocks

Crystal System

Not applicable to a rock; constituent minerals have various systems (e.g., monoclinic, triclinic).

Chemical Formula

Not applicable to a rock; composed of various minerals.

Composition

Primarily plagioclase feldspar (Ca-rich), pyroxene, amphibole, and sometimes olivine. Minor accessory minerals may include magnetite, ilmenite, apatite, and biotite.

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