Rev. Rul. 55-67

Gypsum, in its natural state, is a mineral which may occur in rocklike masses but is very soft and rarely mined or used as stone for ordinary commercial purposes. It is not to be classified as `stone,' as that term is used in section 114(b)(4)(A)(i) of the Internal Revenue Code of 1939 (as amended by section 319 of the Revenue Act of 1951) and section 39.23(m)-5(b) of Regulations 118, unless it is used, or sold for use, by the mine owner or operator as `stone' as defined in such regulations