Why does some limestone discolour or grow moss etc?
Limestone is a porous stone that if kept moist can create a suitable environment for lichen, moss etc to grow. If the growth is not removed in its early stages it may darken the stone. Dark soils, peat etc can also discolour the stone over time if it is not sealed. Sealing the stone with a good sealer such as Waterepel will prevent discolouration.
