Dimensional stones fall into two groups: siliceous or calcareous. The group we see most is calcareous stone, made up of calcium carbonate and sensitive to acid cleaning. This group includes marble, travertine and limestone. Marble is divided into four groups-"A," "B," "C" or "D"-according to soundness and suitability for prospective use. The classification "C" or "D" means the material is comparatively fragile and may need additional fabrication.