How to identify Unidentified Rocks

To correctly identify Unidentified Rocks (Indeterminate Sedimentary/Metamorphic Rocks), 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, ranging from light grey, tan, brown, to dark grey or black, depending on the original composition and any subsequent alteration or incipient metamorphism.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes slightly waxy or sub-vitreous if fine-grained metamorphic minerals are present.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be fine-grained to medium-grained. May show relict clastic textures (e.g., faint bedding, detrital grains) or incipient metamorphic textures (e.g., very fine-grained interlocking crystals, weak foliation, or a massive, hornfelsic texture). The key is the ambiguity – not clearly clastic, crystalline, or foliated.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are typically too small to be seen with the naked eye. May contain microscopic detrital grains (sedimentary) or newly formed metamorphic minerals (e.g., micas, chlorite, quartz, feldspar) that are anhedral or subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   May exhibit a poorly developed parting or fracture, but not a distinct rock cleavage (e.g., slaty cleavage) or mineral cleavage. Fracture is typically irregular or conchoidal.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Often found in transitional zones between sedimentary basins and areas of low-grade regional metamorphism, or in contact metamorphic aureoles where the original rock was sedimentary. Can also be found in areas of intense diagenesis or hydrothermal alteration.

Key Facts

Hardness

Variable, typically 3-6 on Mohs scale, depending on mineralogy and degree of alteration/metamorphism.

Specific Gravity

Variable, typically 2.6-2.8 g/cm³.

Crystal System

N/A (rock, not a single mineral; constituent minerals may have various systems).

Color

Highly variable, often shades of grey, brown, or black.

Luster

Dull, earthy, or sub-vitreous.

Transparency

Opaque.

Fracture

Irregular, sub-conchoidal, or splintery.

Cleavage

Absent or poorly developed parting, not true rock cleavage.

Composition

Highly variable, typically composed of fine-grained quartz, feldspar, clay minerals, micas (muscovite, illite, chlorite), and potentially carbonaceous material or iron oxides. The exact proportions are unknown without analysis.

Physical characteristics

• Crystal Habit: N/A (rock, not a single mineral; constituent minerals are typically anhedral to subhedral, fine-grained).
• Cleavage Type: Absent or poorly developed parting, not true rock cleavage.
• Fracture Type: Irregular, sub-conchoidal, or splintery.
• Tenacity: Brittle.
• Luster Type: Dull to earthy, sometimes sub-vitreous.

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