NOUN (2)

1. a reddish brown sandstone; used in buildings;

2. a row house built of brownstone; reddish brown in color;

The Collaborative International Dictionary of English v.0.48:

Brownstone \Brown"stone`\, n.
   1. A dark variety of sandstone, much used for building
      purposes.
      [1913 Webster]

   2. a building, especially a dwelling, faced with
      brownstone[1].
      [PJC]

WordNet (r) 3.0 (2006):

brownstone
    n 1: a reddish brown sandstone; used in buildings
    2: a row house built of brownstone; reddish brown in color