[Margaret Pilkington]

It was a Donkey stone.........and they came in different colours......cream, sand and a sort of donkey grey....which might be why they are called donkey stones.
You could buy them from hardware stores, or the rag man would give them to you if you gave him jam-jars, bottles or rags(yes we were re-cycling way back then).
I am sure the Stall that was in Accrington Market hall(Matthews) had them for sale right up until they refurbished the market hall........I can't say I have seen any recently though...they seem to have gone upmarket.