These doorways are the best doorways in Birmingham and that is a FACT. The black surrounds are a type of limestone, called Frosterly Marble, which is extracted in Durham (near Frosterly). The white marks are fossils of mostly corals, some of them are horizontal cross sections (the circles) while some are longitudinal sections (the long worm shapes). This stone is Carboniferous in age, so around 350 million years old. The shapes of these coral fossils tell us that during their growth stage, they were knocked over – these types of coral prefer to grow vertically, so any changes in direction tell us they fell over, grew straight, got knocked over, and grew straight again.
You can read more in Part One of the Birmingham Urban Geology guide.