I bought this recently in Agadir for only $25 so I'm happy whatever it is. It is said to be a stromatolite, though it may be just a desert rose of some type, though it's very hard and heavy, definitely not gypsum or barite. Some sites on the net suggest precambrian, ranging from 2.6 down to .6 billion years old, depending where you look. Some say Cambrian, some Ordovician and others Devonian or Cretaceous. Any ideas anyone?