Limestone is a porous sedimentary rock made up of layers of calcium carbonate formed from the accumulation of organic matter in shallow seas, which is then turned to stone by millions of years of pressure. Limestone can come in a variety of density and hardness and some are as tough as granite making an extremely hard wearing and durable paving stone.
As a surface it can stain quite easily. The risk of potential stains can be largely averted by impregnating or sealing the surface of the limestone. Before doing this, make sure the surface is very clean.