TUFO, a kind of stone, according to M. Raspe, formed of volcanic ashes ferruminated and concreted together. It is of various colours; blackish-grey, bluish-grey, and yellow; every colour having a different mixture and solidity: but all of them have the bad quality of mouldering down on long exposure to the weather; notwithstanding which, they have been used in buildings both ancient and modern. The yellow kind resists the air less than any other.