Creating props - 'stone blocks' for medieval play

Time again, to ask the experts...can anyone advise how to make /obtain stone blocks strong enough to hold 1 or 2 persons (sitting on them). I've seen several productions using similar, but I was told they were fibreglass?? We found and used a large polystyre block some years ago, that worked well, but am not sure where one would obtain similar. The poly block was approx 900MM L X 400MM H x 450MM W..... Johnny Grim