Ahhh… the streets of Old San Juan – what a wonderful blue glow to them.
Rather than being cut from stone or cast as bricks, these cobblestones were an ingenious use of slag from Spain’s iron foundries (slag is iron waste when refined, and was usually piled in huge slagheaps at the foundries). When it was cast into blocks, the slag made long lasting cobblestones. The stones were then brought to the island as ballast in ships in the 16th century. Their blue tint gives the streets of Old San Juan a very colorful look.