Limestone

Hardness: 3-4 (Mohs), Color: Grey, white, or tan with pitted texture, Luster: Dull/Earthy, Crystal Structure: Hexagonal (calcite), Cleavage: Perfect rhombohedral in calcite crystals, SG: 2.5-2.7

Formed primarily in marine environments from the accumulation of shell, coral, algal, and fecal debris. It can also form through chemical precipitation from spring or ocean water. Many specimens date back millions of years to the Paleozoic or Mesozoic eras.

Used extensively in construction as road base, concrete aggregate, and building stone. It is also used as an industrial flux in steel manufacturing, agricultural lime to neutralize soil acidity, and as a source of calcium in animal feed.

Limestone makes up about 10% of the total volume of all sedimentary rocks. It is the primary rock type in which most of the world's caves are found, formed over thousands of years as acidic rainwater dissolves the rock.

Identify in the field by testing with cold dilute hydrochloric acid (it will fizz/effervesce). Look for pitted weathering and fossil inclusions. Common in areas that were once ancient sea beds. Tips: check for biological remains like crinoid stems or shell imprints.