The Blue Cobblestones of Old San Juan

These cobblestones aren't stone at all. 
They are made of slag, the waste when iron is refined. The stones were brought to the island as ballast in ships from Spain in the 16th century. The ballast was unloaded then the ships were filled with Puerto Rico's sugar cane. Instead of being discarded the cobblestones were used as street pavers. 
I've read that the molds used to shape them in Spain were the sames molds they used to make gold bars. 
The iron gives the stones their color which shifts depending upon the heat, sun and humidity, or so I'm told. I plan on testing this out for myself in the future.