How to identify River Rock

To correctly identify River Rock (Sedimentary or Metamorphic Rock (e.g., Quartzite, Sandstone, Gneiss)), 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

   Extremely variable, reflecting the source rock. Can be white, grey, black, brown, red, green, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, but can be vitreous (glassy) if composed predominantly of quartz or chert, or if wet.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, often polished surface due to abrasion. Sub-rounded to well-rounded shapes. Aphanitic to phaneritic, depending on the clast composition.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the rock as a whole, as it is a collection of clasts. Individual mineral grains within the clasts will exhibit their characteristic crystal forms if visible.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the rock as a whole. Individual mineral grains within the clasts may exhibit cleavage (e.g., feldspar, mica).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial environments, including riverbeds, floodplains, terraces, and alluvial fans. Also found in ancient sedimentary deposits (paleo-rivers) where they form conglomerates.

Key Facts

Hardness

Varies widely, from 1 (talc) to 7 (quartz) or higher, depending on the hardest mineral present in the clasts. Often dominated by quartz (Mohs 7).

Specific Gravity

Varies widely, typically 2.5 to 3.0 g/cm³, depending on the density of the constituent rock types.

Crystal System

Not applicable to the rock as a whole. Individual mineral grains within the clasts will have their characteristic crystal systems.

Color

Highly variable, reflecting the source rocks.

Luster

Dull to earthy, sometimes vitreous.

Transparency

Opaque to translucent, depending on the clast composition.

Fracture

Conchoidal (if quartz/chert), irregular, or splintery, depending on the clast composition.

Cleavage

Not applicable to the rock as a whole. Individual mineral grains may exhibit cleavage.

Composition

Polymictic; composed of fragments of various igneous, metamorphic, and sedimentary rocks and minerals (e.g., quartz, feldspar, mica, amphibole, pyroxene, chert, granite, basalt, gneiss, sandstone, limestone).

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Individual mineral grains within clasts exhibit their own habits.
• Cleavage Type: Not applicable to the rock as a whole. Cleavage is present in individual mineral grains within the clasts (e.g., perfect in mica, good in feldspar).
• Fracture Type: Variable, depending on the clast composition (e.g., conchoidal for quartz, irregular for many other rocks).
• Tenacity: Brittle, but overall strength depends on the weakest clast and any cementing material if consolidated.
• Luster Type: Dull, earthy, or vitreous.

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