Chicago Common Brick
Much of Chicago is built from Chicago Common bricks. If you're in Chicago, you're probably near some now. Chicago Commons are the rougher and dirtier bricks on the sides and back of many of Chicago's buildings. They're made from the clay from the Chicago River and when fired they can turn a range of colors, like buff yellow, salmon pink, or deep red. They age beautifully and take on a beautiful patina.
When originally laid, Chicago Commons were tucked away on the back and sides of buildings, playing second fiddle to the cleaner and crisper face brick on the front of the building. Chicago Commons haven't been made since the 80s and are now in high demand as reclaimed bricks for new construction and renovations.
The best writing that exists about Chicago Commons and the culture that surrounds them can be found in this writeup by the Chicago Reader.
One of the cooler things about Chicago Commons is how unique they are to Chicago and the clay deposits here. Move up the lake to Milwaukee and the clay is different enough that you get Cream City Brick.