Length-slow chalcedony, having its c -axis parallel to the fibers, is unusual in nature. Occurrences so far investigated reveal that this rare type of optically fibrous silica occurs almost exclusively in association with sulphates and evaporites. Often the evaporite minerals have been totally removed, and thus the survival of resistant length-slow chalcedony reveals the presence of former salt-flat, sabkha or sulphate-rich environments where none were before suspected. This type of chalcedony also in some instances forms in semi-arid, alkaline soils. Discovery of this simple criterion has led to development of an additional series of characteristics for identification of silicified evaporites. All siliceous deposits should be re-examined for presence of this important genetic marker.