trass

n. [ D. tras or Gr. trass, probably fr. It. terrazzo terrace. See Terrace. ] (Geol.) A white to gray volcanic tufa, formed of decomposed trachytic cinders; -- sometimes used as a cement. Hence, a coarse sort of plaster or mortar, durable in water, and used to line cisterns and other reservoirs of water. [ Formerly written also tarras, tarrace, terras. ] [ 1913 Webster ]

☞ The Dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine. [ 1913 Webster ]