sedimentary

Limestone (specifically Iron-stained or Argillaceous Limestone)

Calcium Carbonate (CaCO3) with Iron Oxide (Fe2O3) impurities

Hardness: 3 on Mohs scale; Color: Tan, brown, or rusty orange due to iron oxidation; Luster: Dull to earthy; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.3–2.7

Hardness: 3 on Mohs scale
Color: Tan, brown, or rusty orange due to iron oxidation
Luster: Dull to earthy

Identified May 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of shell, coral, algal, and fecal debris in clear, warm, shallow marine waters. These specific specimens show significant weathering and iron staining from groundwater exposure.

Uses & applications

Primary source for cement, road base/aggregate, agricultural lime to neutralize soil acidity, and as a flux in steel making.

Geological facts

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary rock type found in karst landscapes, which feature caves and sinkholes.

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 prehistoric seabeds.

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