vitriol stone

vitriol stone

[′vi·trē·əl ‚stōn]
(mineralogy)
A hard, crystalline material, mainly a mixture of ferric sulfate and aluminum sulfate, that is extracted from weathered pyritic schist and used in the manufacture of sulfuric acid.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.