The chip on the top centre of the slate, means nothing..its natural, if the chip was on the actual painting, then it would be virtually worthless, as for being 100 year old slate...add a few zero,s to that, it may have come off a roof 100 years ago, but,..as slate is quarried, then like all fossil rock.its millions of years old,..over here in UK,there are primarily.3 types of slate, that are used to slate roofs, 1 being Ballachulish,,which is Scottish,then there,s Welsh, then Westmorland....2 of these kind, are smooth,ish, whilst the Scotty, is rougher,..see image of Welsh