That African rock is pictured on the lithophone page I linked to. Looks like it must be getting smaller and higher pitched over the centuries as bits get chipped off it.

I can't find a reference now, but there was a collection of lithophonic schist plates found in south-west Scotland a while ago that seem to have been intended as a set (five notes, I think). Maybe 3000 or 4000 years old? They might still be playable but I don't think they have been played since being dug up.