|a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison.|
|a screen or mat covered with a dark material for shielding a camera lens from excess light or glare.|
|limestone (līm'stōn') Pronunciation Key
A sedimentary rock consisting primarily of calcium carbonate, often in the form of the minerals calcite or aragonite, and sometimes with magnesium carbonate in the form of dolomite. Minor amounts of silica, feldspar, pyrite, and clay may also be present. Limestone can occur in many colors but is usually white, gray, or black. It forms either through the accumulation and compaction of fossil shells or other calcium-carbonate based marine organisms, such as coral, or through the chemical precipitation of calcium carbonate out of sea water.
Sedimentary rock formed primarily of calcium carbonate, often the skeletons of small marine organisms.