Limestone

Hardness: 3 on Mohs scale; Color: Grey, tan, or white; Luster: Dull to earthy; Crystal structure: Hexagonal (as calcite crystals); Cleavage: Perfect rhombohedral; Specific Gravity: 2.5-2.7

Formed primarily from the accumulation of shell, coral, algal, and fecal debris in marine environments through a process of lithification. It can also form chemically through the precipitation of calcium carbonate from water. Found in geological formations ranging from the Paleozoic to the Cenozoic eras.

Extensively used as a building material, in the manufacturing of cement, as an aggregate for road bases, in agriculture to neutralize soil acidity, and as a flux in steel making. Small fragments are common for gravel paths.

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary rock type found in karst landscapes, leading to the formation of extensive cave systems globally.

Identify in the field by testing with a drop of dilute hydrochloric acid (it will fizz or effervesce). It is commonly found in ancient seabed locations and is often identifiable by the presence of small marine fossils. Look for it in layers or beds in quarries or natural outcrops.