Dictionary.net

• A variety of shale or clay slate, containing iron pyrites, the decomposition of which leads to the formation of alum, which often effloresces on the rock.

2010 - New Age Dictionary Database
By Oddity Software

• A variety of shale or clay slate, containing iron pyrites, the decomposition of which leads to the formation of alum, which often effloresces on the rock.

1913 - Webster's Revised Unabridged Dictionary
By Noah Webster.