How to identify Granite and Basalt Pebble

To correctly identify Granite and Basalt Pebble (Granite (felsic intrusive igneous rock) and Basalt (mafic extrusive igneous rock)), 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

   Granite: Typically light-colored (pink, white, grey, often speckled). Basalt: Typically dark-colored (black, dark grey, dark green).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Granite: Vitreous to dull (due to weathering) on mineral grains. Basalt: Dull to sub-vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Granite: Phaneritic (coarse-grained, interlocking crystals visible to the naked eye). Basalt: Aphanitic (fine-grained, crystals generally not visible without magnification) or porphyritic (larger crystals in a fine-grained matrix), sometimes vesicular (containing gas bubbles). Pebbles will have a smooth, rounded surface due to abrasion.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Granite: Anhedral to subhedral interlocking crystals of quartz, feldspar, and mica. Basalt: Microscopic crystals, sometimes phenocrysts of olivine or pyroxene.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Granite: Individual mineral components (feldspar, mica) exhibit cleavage. Quartz has no cleavage. Basalt: Mineral components (pyroxene, plagioclase) exhibit cleavage, but often not visible in fine-grained rock.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granite: Formed in continental crust, often associated with mountain building and subduction zones. Basalt: Formed in oceanic crust, volcanic arcs, hot spots, and continental rifts. Pebbles of both are found in fluvial (river), lacustrine (lake), glacial, and marine (beach) environments.

Key Facts

Hardness

Granite: 6-7 (Mohs, due to quartz and feldspar). Basalt: 6-7 (Mohs, due to pyroxene and plagioclase).

Specific Gravity

Granite: 2.6-2.7 g/cm³. Basalt: 2.8-3.0 g/cm³.

Crystal System

Granite: Individual minerals (quartz, feldspar) are typically trigonal and triclinic/monoclinic, respectively. Basalt: Individual minerals (pyroxene, plagioclase) are monoclinic and triclinic, respectively.

Color

Granite: Light-colored (pink, white, grey, often speckled). Basalt: Dark-colored (black, dark grey, dark green).

Luster

Dull to sub-vitreous.

Transparency

Opaque.

Fracture

Granite: Irregular to conchoidal (quartz). Basalt: Conchoidal to irregular.

Cleavage

Granite: Present in feldspar (2 directions at ~90°) and mica (1 perfect basal). Basalt: Present in pyroxene (2 directions at ~90°) and plagioclase (2 directions at ~90°), but often not visible due to fine grain size.

Composition

Granite: Predominantly quartz (>20%), alkali feldspar, plagioclase, and micas (biotite, muscovite). Basalt: Predominantly plagioclase feldspar, pyroxene, and often olivine; minor amphibole, magnetite.

Physical characteristics

• Crystal Habit: Granite: Granular, interlocking anhedral to subhedral crystals. Basalt: Aphanitic (microcrystalline) to porphyritic, sometimes vesicular.
• Cleavage Type: Granite: Good in feldspar, perfect in mica. Basalt: Good in pyroxene and plagioclase, but rarely observed in hand samples due to fine grain size.
• Fracture Type: Granite: Irregular to conchoidal (quartz). Basalt: Conchoidal to irregular.
• Tenacity: Brittle.
• Luster Type: Dull to sub-vitreous.

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