sedimentary

Limestone

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: typically white, gray, or tan; Luster: dull to earthy; Crystal structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific gravity: 2.5-2.7.

Hardness: 3 on Mohs scale
Color: typically white, gray, or tan
Luster: dull to earthy

Identified May 13, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: typically white, gray, or tan; Luster: dull to earthy; Crystal structure: Trigonal (as calcite); Cleavage: Perfect rhombohedral; Specific gravity: 2.5-2.7.

Formation & geological history

Formed most commonly in clear, warm, shallow marine waters from the accumulation of shell, coral, algal, and fecal debris. It can also be chemical in origin (precipitated from water). It can range from Precambrian to Holocene in age.

Uses & applications

Used extensively in construction as road base, railroad ballast, and an aggregate in concrete. It is a key ingredient in cement, lime, and is used to neutralize acid in soil and water.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary building block of the Great Pyramids of Giza and most of the world's most spectacular caves are carved into limestone.

Field identification & locations

Identify in the field by applying a drop of dilute hydrochloric acid; it will effervesce (fizz) vigorously. It is commonly found in areas that were once ancient sea beds. Check for fossil remains which are common in many varieties.