normal roof slate or quarry slate is fine. I know of people here, who collect slate from the beach, as long as its clean you should be ok. A quick test, I was told a while back was to dip a piece of stone into vinegar and if it fizzes, do not use.
normal roof slate or quarry slate is fine. I know of people here, who collect slate from the beach, as long as its clean you should be ok. A quick test, I was told a while back was to dip a piece of stone into vinegar and if it fizzes, do not use.