Like dolomite and chalk, limestone is a flux stone which forms entire sedimentary layers. In addition, limestone layers are the only sedimentary layers to contain galena or malachite.
In general, limestone layers tend to yield more ores and gems per volume mined than those of other stones, which makes them a prime candidate for exploratory mining. Any overburden thus generated will also be doubly valuable compared to regular stone for crafting as a result of limestone's flux property.
"Limestone" is also the name of the seventh month of the dwarven calendar, covering early Autumn.
In Real Life
Limestone is a sedimentary rock composed largely of minerals like calcite. It can also contain silica in the form of chert, and varying amounts of clay, silt and sand carried in by rivers.