How to identify Weathered Rock with Algae/Lichen

To correctly identify Weathered Rock with Algae/Lichen (Indeterminate Rock (covered with biological growth)), 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

   Highly variable, dominated by the color of the biological growth: typically shades of green (algae, young lichens), gray, white, yellow, orange, or black (various lichen species, cyanobacteria). The underlying rock color is often obscured.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, imparted by the biological growth. The original rock's luster is usually not visible.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough, granular, crusty, or powdery, depending on the type of biological growth. Lichens can form flat crusts (crustose), leafy structures (foliose), or branched forms (fruticose). Algae may appear as slimy films or powdery coatings.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable, as this describes a surface phenomenon on an existing rock. The underlying rock will have its own crystal form if crystalline.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable, as this describes a surface phenomenon. The underlying rock will exhibit its own cleavage properties.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Ubiquitous across all terrestrial environments where rocks are exposed to light and moisture, from polar regions to deserts, high mountains to coastal areas. Common on outcrops, boulders, gravestones, and man-made structures. The specific type of algae or lichen will depend on climate, humidity, light exposure, and the chemical composition of the underlying rock.

Key Facts

Hardness

Variable, generally very soft (biological material). The underlying rock will have its own hardness.

Specific Gravity

Variable, generally low (biological material). The underlying rock will have its own specific gravity.

Crystal System

Not applicable (biological growth on a rock).

Color

Green, gray, yellow, orange, black (from algae/lichen).

Luster

Dull to earthy.

Transparency

Opaque (biological growth).

Fracture

Not applicable (biological growth).

Cleavage

Not applicable (biological growth).

Composition

Organic compounds (carbohydrates, proteins, lipids) from algae/lichen, plus inorganic components of the underlying rock.

Physical characteristics

• Crystal Habit: Not applicable (biological growth).
• Cleavage Type: Not applicable (biological growth).
• Fracture Type: Not applicable (biological growth).
• Tenacity: Brittle to flexible (biological growth).
• Luster Type: Dull to earthy.

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