Brownstone is a type of sandtone rock, generally of Hummelstown brownstone origin in the historic Washington DC neighborhoods. The word brownstone is often used to denote a type of building, but in most cases they are just late century rowhomes.
At the following link, you can learn a lot more about the history, causes of deterioration and restoration of historic brownstone.
This particular building is built with ashlar masonry.
See the following link for more information on sandstone rock and stone masonry.