Mineral materials.
Mineral materials. A collective term used throughout this subpart to describe petrified wood and common varieties of sand, gravel, stone, pumice, pumicite, cinders, clay, and other similar materials. Common varieties do not include deposits of those materials which are valuable because of some property giving them distinct and special value, nor do they include so-called block pumice which occurs in nature in pieces having one dimension of two inches or more and which is valuable and used for some application that requires such dimensions.