Lava millstone

Lava \La"va\ (l[aum]"v[.a]; 277), n. [It. lava lava, orig. in
   Naples, a torrent of rain overflowing the streets, fr. It. &
   L. lavare to wash. See {Lave}.]
   The melted rock ejected by a volcano from its top or fissured
   sides. It flows out in streams sometimes miles in length. It
   also issues from fissures in the earth's surface, and forms
   beds covering many square miles, as in the Northwestern
   United States.
   [1913 Webster]

   Note: Lavas are classed, according to their structure, as
         scoriaceous or cellular, glassy, stony, etc., and
         according to the material of which they consist, as
         doleritic, trachytic, etc.
         [1913 Webster]

   {Lava millstone}, a hard and coarse basaltic millstone from
      the neighborhood of the Rhine.

   {Lava ware}, a kind of cheap pottery made of iron slag cast
      into tiles, urns, table tops, etc., resembling lava in
      appearance.
      [1913 Webster]