The Power Stone is imbued with a powerful magical invocation. When it is held out by a Wizard before he casts a spell, the effect is to enhance the efficacy of the magic. A further two dice are added to the Casting roll. Note that using a Power Stone will allow a Wizard to use more Power dice than he is normally permitted. For example, a First Level Wizard may read a Power Stone and thus use four Power dice to cast a spell (2 basic + 2 from a Power Stone). A Power Stone can only be used once - after one use its power is exhausted.
Using a Power Stone will bring any spell in play cast earlier by that Wizard to an end in the same way as casting a new spell by ordinary means.
As with scrolls, Power Stones are not unique items - they are prepared by a Wizard prior to battle and it is quite possible for several Wizards to carry Power Stones, and for a Wizard to carry more than one. However, only one Power Stone can be used to enhance a spell.
Note that a spell cast with a Power Stone can never be cast with Irresistible Force, though it can be Miscast.