Lava millstone

a hard and coarse basaltic millstone from the neighborhood of the Rhine.

See also: Lava

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
Water-mills were powered by leats of water carried down oak-lined shoots to drive horizontal wheels attached to lava millstones, the lava imported from the Continent because of its lightness and hardness.