Limestone

Physical Properties

Hardness: 3 (Mohs scale); Color: White, tan, cream, or light gray; Luster: Dull to earthy; Crystal structure: Trigonal (microcrystalline calcite); Cleavage: None (rhombohedral in calcite crystals but often invisible in massive rock); Specific Gravity: 2.3–2.7

Formation & Geological History

Formed primarily through the accumulation of shell, coral, algal, and fecal debris in marine environments. It can also form chemically through the precipitation of calcium carbonate from lake or ocean water. Significant deposits range from the Paleozoic era to the present.

Uses & Applications

Extensively used in construction as an aggregate for road bases, a primary ingredient in Portland cement, a flux in steel making, and as a soil conditioner in agriculture to neutralize acidity.

Geological Facts

Limestone is the primary rock type found in karst landscapes, leading to the formation of spectacular cave systems like Mammoth Cave. Over 10% of all sedimentary rocks are limestones.

Field Identification & Locations

Identify in the field by its softness (can be scratched by a copper penny or steel knife) and its strong effervescence (bubbling) when a drop of diluted hydrochloric acid is applied. Many specimens contain visible small fossils or shell fragments.

Identified on: 4/25/2026

Mode: Standard