According to Marbodaeus (10351123), 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 mans mouth.
Leonarduss 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.