Where scoria is not available, a lightweight aggregate can be produced by heating shale in a rotating kiln under controlled conditions. With the proper type of shale, the material will have the properties, appearance, and vesicles of scoria. It is sold under the name “expanded aggregate,” “expanded clay,” or “grow rocks” and used for the same purposes as crushed scoria.

Author’s note: “I am usually a cheerleader for the mining industry, but I will admit that it troubled me to see a cinder cone being removed to produce aggregate.”