Limestone (specifically Marl or Chalk)

Physical Properties

Hardness: 3 (Mohs scale); Color: White, light gray; Luster: Dull/Earthy; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (in bulk form); Specific Gravity: 2.3–2.7

Formation & Geological History

Formed through the accumulation of shell, coral, algal, and fecal debris in marine environments. This specific crumbly white specimen suggests a biological origin from marine microorganisms.

Uses & Applications

Primarily used in construction (as aggregate), cement manufacturing, soil conditioning in agriculture, and as a precursor for quicklime.

Geological Facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is essential for the formation of cave systems through the process of chemical weathering by slightly acidic rainwater.

Field Identification & Locations

Identify in the field by its softness (can be scratched by a copper coin) and its energetic effervescence (bubbling) when a drop of weak acid like vinegar is applied. Commonly found in road beds as crushed gravel.

Identified on: 4/17/2026

Mode: Standard