Diamond of Summoning

In the RPG
This item appears to be an ordinary (if extremely valuable) daimond in all ways, except that a faint flickering light emanates forth from within. The gem may be, but is not always, mounted on some other piece of jewelry, such as a ring or amulet. Items of this sort are often given to a knight or similar character sent upon a quest, with instructions not to invoke the gem's powers her mission is complete. When the command word is spoken the user will be transported to the original owner.