For marble, limestone and sandstone, use water and soft bristle brushes, natural or nylon . Granted it doesn't sound like a miracle cleaner and it doesn't happen quickly but with some patience and time this will clean most environmental soiling and lichens from stones. Keep the stone wet at all times; really wet. Where a garden hose is not available, be sure to bring plenty of jugs of water and keep dowsing the stone as you work and, most importantly, flush the stone well when done. Scrub the stone from the bottom up to avoid streaking and further staining.
Regards Tom
