The cobblestone (Adoquines) streets of Old San Juan are made of blue stone cast from furnace slag carried here as ballast aboard Spanish ships. The distinctive blue hue is caused by age and dampness. Although there are other colors of stone used for paving, such as red brick and black flagstone, the blue ones are most common. There are two types of stones used in street paving: macadamoid and adoquine. Macadamoid is a hard rock that is smoothed down to make a road surface. Adoquine is the name given to any type of gravel or small rock. These are the main ingredients in asphalt mixes that are used to pave roads outside of cities and towns. In addition, there are various types of crushed rock that are used for erosion control and decorative purposes. Macadamoid was developed in Scotland in the early 1800s and came to America when the first highways were built here. It's made up of small pieces of stone or gravel mixed with tar or hot bitumen (the same substance used to build roads today). The stone acts as a binder for the tar or bitumen and helps it adhere to the ground so it doesn't wash away during rainstorms.