According to Marbodaeus (1035-1123), this gem resembled the beryl in its properties and was most valuable in divination. It served for the invocation of spirits, and oracular responses could be discovered in it. Albertus Magnus called it "diacodos," and it is possibly to this stone that Braithwaite alludes in English Gentleman: "For as the precious stone Diacletes, though it have many rare and excellent sovereignties in it, yet loseth them all if put in a dead man's mouth."
Leonardus's remarks about the "Diacodas" or "Diacodus" are too curious to omit: "It disturbs devils beyond all other stones, for, if it be thrown in water, with the words of its charm sung, it shows various images of devils, and gives answer to those that question it. Being held in the mouth, a man may call any devil out of hell, and receive satisfaction to such questions as he may ask."
"Diadochus." Encyclopedia of Occultism and Parapsychology. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/diadochus
"Diadochus." Encyclopedia of Occultism and Parapsychology. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/diadochus