Limestone is defined as a rock of sedimentary origin composed principally of calcium carbonate or the double carbonate of calcium and magnesium, or a combination of these two minerals. Recrystallized limestone, compact microcrystalline limestone, and travertine that are capable of taking a polish are promoted, marketed and sold as either limestone or marble, particularly in the United States. Dimension limestone is divided into three sub-classifications that describe their densities in approximate ranges, as follows:
- LOW DENSITY – Limestone having a density ranging from 110 through 135/lb/ft3 (1760 through 2160 kg/m3).
- MEDIUM DENSITY – Limestone having a density greater that 135 and not greater than 160 lb/ft3 (2160 through 2560 kg/m3).
- HIGH DENSITY – Limestone having a density greater than 160 lb/ft3 (2560 kg/m3).
Limestone contains a number of distinguishable natural characteristics, including calcite streaks or spots, fossils or shell formations, pit holes, reedy formations, open texture streaks, honeycomb formations, iron spots, travertine-like formations and grain formation changes. One or a combination of these characteristics will affect the texture.