http://archaeologyinbulgaria.com/2021/03/30/fine-marbles-14-different-colors-from-constantine-the-greats-danube-bridge-opening-in-328-ad-found-in-roman-city-ulpia-oescus-in-north-bulgaria/
Fine Marbles in 14 Different Colors from Constantine the Great’s Danube Bridge Opening in 328 AD Found in Roman City Ulpia Oescus in North Bulgaria