Fossiliferous Limestone (Chertified Limestone)

Physical Properties

Hardness: 3-4 (if limestone) to 7 (if chertified); Color: Tan, beige, light brown; Luster: Dull, earthy; Crystal structure: Amorphous/Cryptocrystalline; Conchoidal fracture in silica-rich areas; Visible small crinoid or shell impressions.

Formation & Geological History

Formed in warm, shallow marine environments where skeletal fragments of marine organisms (like corals or mollusks) accumulated. Over millions of years, pressure and chemical cementation lithify these remains into rock. Often found in Paleozoic Era strata.

Uses & Applications

Commonly used in the manufacture of cement, as a crushed stone for road base, and as a soil conditioner. Fossil-rich specimens are valued as educational tools or decorative garden stones.

Geological Facts

This specimen appears to have 'vugs' or small cavities where fossils may have dissolved or been replaced by silica. Indiana (USA) is famous for its high-quality Mississippian-age fossiliferous limestone used extensively in iconic architecture.

Field Identification & Locations

Identify in the field by looking for visible fossil imprints and testing with dilute hydrochloric acid; if it fizzes, it's limestone. If it's too hard to scratch with a steel nail, it's likely been silicified (turned to chert). Found in widespread sedimentary basins.

Identified on: 4/13/2026

Mode: Standard