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Limestone is a type of rock composed of a large volume of calcite crystals. Mineralogically, calcite and limestone are essentially the same thing, so its just the volume that defines limestone as a rock instead of a mineral.

Limestone is one of the few rocks that is composed of only one mineral.

A rock is an aggregate of different minerals. A rock, such as limestone, made from only 1 mineral is monomineralic. Dunnite, Clinopyroxenite and Anorthosite are other examples of olivine, clinopyroxene and anorthiote, respectively.

So 1 calcite crystal is a calcite crystal. A grouping of servel crystals is a chunk of limestone, travertine, chalk, tufa or marble, depending on how it formed and whether it has been metamorphosed.

Limestone is a sedimentary rock that comes from calcite a mineral. The primary source is from shells of marine animals that accumulated in ancient inland seas. Over millions of years the pressure from overlying layers compacts the sediment with enough pressure to cause them to harden and become rock-like. Also, crystallization occurs as the oceans ot inland seas dried up. This latter process caused limestone (Calcium carbonate) to form limestone rocks.