How to identify Lichen-covered rock

To correctly identify Lichen-covered rock (Rock with biological growth (lichen)), 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, determined by the lichen species and underlying rock. Lichens can be green, yellow, orange, red, brown, gray, black, or white. The underlying rock color may be visible in patches or through translucent lichen thalli.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Not applicable to the composite entity. The rock itself will have its inherent luster (e.g., vitreous, dull, earthy), while the lichen growth typically appears matte or dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, depending on the lichen growth form. Crustose lichens create a rough, granular, or smooth crust. Foliose lichens have a leafy, often wrinkled or lobed texture. Fruticose lichens are shrubby or hair-like. The underlying rock texture may be obscured or modified by the lichen.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable. Lichens do not form crystals. The underlying rock may exhibit crystalline forms if it is an igneous or metamorphic rock.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable. Lichens do not exhibit cleavage. The underlying rock may or may not have cleavage planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Ubiquitous in almost all terrestrial environments where rocks are exposed to light and moisture, from polar regions to deserts, high altitudes to sea level. Common on outcrops, boulders, cliffs, gravestones, and man-made structures. Requires stable rock surfaces and sufficient atmospheric moisture.

Key Facts

Hardness

Not applicable to the composite entity. The hardness is that of the underlying rock (e.g., quartz 7, calcite 3). Lichens themselves are soft and pliable.

Specific Gravity

Not applicable to the composite entity. The specific gravity is that of the underlying rock (e.g., granite ~2.6-2.7, limestone ~2.7).

Crystal System

Not applicable. Lichens are biological organisms. The underlying rock may have a specific crystal system if it is crystalline.

Color

Highly variable, depending on lichen species and underlying rock.

Luster

Dull to matte (lichen), variable for underlying rock.

Transparency

Opaque (lichen), variable for underlying rock.

Fracture

Not applicable to lichen. The underlying rock will exhibit its characteristic fracture (e.g., conchoidal, irregular).

Cleavage

Not applicable to lichen. The underlying rock may or may not exhibit cleavage.

Composition

Composite of organic matter (lichen thallus, primarily fungal hyphae and algal/cyanobacterial cells) and inorganic rock minerals (e.g., silicates, carbonates, oxides, depending on the rock type).

Physical characteristics

• Crystal Habit: Not applicable (biological growth).
• Cleavage Type: Not applicable (biological growth).
• Fracture Type: Not applicable (biological growth).
• Tenacity: Not applicable (biological growth).
• Luster Type: Dull to matte (from lichen growth).

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