How to identify Limestone

To correctly identify Limestone (Limestone), 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 buff. Can also be black, pink, or yellow 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 can be vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, from very fine-grained (aphanitic) to coarse-grained (phaneritic). Can be clastic (composed of shell fragments, ooids) or non-clastic (micritic, crystalline). Often feels slightly gritty or smooth.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   As a rock, it does not have a single crystal form. The constituent mineral calcite typically forms rhombohedral crystals, but in limestone, it often occurs as microcrystalline grains or shell fragments.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   The constituent mineral calcite exhibits perfect rhombohedral cleavage in three directions. In the rock, this is usually not observable unless it is coarsely crystalline.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

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

Key Facts

Hardness

3 on the Mohs scale (due to calcite)

Specific Gravity

2.71 g/cm³ (for pure calcite; limestone varies with impurities, typically 2.6-2.8)

Crystal System

Trigonal (for constituent calcite)

Color

White, gray, tan, buff, black, pink, yellow

Luster

Dull, earthy, vitreous

Transparency

Opaque to translucent (for individual calcite crystals)

Fracture

Conchoidal to uneven (for individual calcite crystals; rock fracture is typically irregular)

Cleavage

Perfect rhombohedral (for constituent calcite)

Composition

Predominantly calcium carbonate (CaCO3), usually as calcite. May contain dolomite, quartz, clay minerals, iron oxides, and organic matter.

Physical characteristics

• Crystal Habit: Microcrystalline to macrocrystalline aggregates; often clastic (bioclastic, oolitic).
• Cleavage Type: Perfect rhombohedral (of constituent calcite).
• Fracture Type: Irregular to subconchoidal (of the rock).
• Tenacity: Brittle.
• Luster Type: Dull, earthy, sometimes vitreous.

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