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crystallomancy (n.)

"divination by means of crystals," 1610s; see crystal + -mancy.

The operator first muttered over the crystal (a beryl was preferred) certain formulas of prayer, and then gave it into the hands of a young man or a virgin, who thereupon, by oral communication from spirits in the crystal, or by written characters seen in it, was supposed to receive the information desired. [Century Dictionary]