How to identify Limestone

To correctly identify Limestone (Limestone (sedimentary rock composed primarily of calcium carbonate)), 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

   Typically white, gray, tan, or light brown. Can be black, yellow, or pink due to impurities (e.g., organic matter, iron oxides).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in most varieties; crystalline varieties may exhibit a vitreous (glassy) luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable. Can be fine-grained (micritic), medium-grained (calcarenite), coarse-grained (calcirudite), clastic (coquina), or porous (travertine, tufa). Often feels smooth to slightly gritty.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual calcite crystals are typically microscopic in most limestones. Macroscopic crystals may be present as cement or in veins. Calcite itself forms rhombohedral crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Calcite, the primary mineral, exhibits perfect rhombohedral cleavage in three directions, though this is often not visible in fine-grained limestone.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Shallow marine environments (continental shelves, carbonate platforms), deep marine environments (pelagic limestones), lacustrine (lake) environments, caves (speleothems), and around hot or cold springs (travertine, tufa).

Key Facts

Hardness

3 on the Mohs scale (for calcite)

Specific Gravity

2.71 g/cm³ (for pure calcite)

Crystal System

Trigonal (for calcite)

Color

White, gray, tan, light brown; can be black, yellow, pink, or red due to impurities.

Luster

Dull to earthy; vitreous in crystalline varieties.

Transparency

Opaque to translucent (rarely transparent in pure, large calcite crystals).

Fracture

Conchoidal to uneven (for calcite); often irregular in rock mass.

Cleavage

Perfect rhombohedral (3 directions at 74° and 106°) for calcite, but often not visible in fine-grained limestone.

Composition

Primarily calcium carbonate (CaCO3), typically as calcite. May contain minor amounts of aragonite, dolomite, quartz, clay minerals, iron oxides, and organic matter.

Physical characteristics

• Crystal Habit: Typically massive, granular, or cryptocrystalline. Individual calcite crystals are rhombohedral.
• Cleavage Type: Perfect rhombohedral (for calcite).
• Fracture Type: Conchoidal to uneven (for calcite); irregular in rock mass.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, or vitreous.

Explore Limestone