Everything has its price, and as the Stone Giveth, the Stone Taketh Away.
The Zero-Sum Stone (or 0SS) is almost identical to the original Stone, but it has one additional rule: Every time the 0SS grants something legally wished on it, it takes away something else of equivalent value from somewhere else within it's Range. If, say, Jon wished for a sandwich, then the ingredients for the sandwich the 0SS makes would subsequently disappear from his fridge. If Karyn wished for Sarah's blonde hair and big boobs, then Sarah would turn into a redhead with a flat chest.
Note that this is not necessarily the usual Conservation of Mass or Energy, or Equivalent Exchange in the usual sense: the 0SS is also capable of swapping around, with no total global change, abstract and mostly intangible quantities, like Money, Intelligence, Personalities, or Magical Qualities. If Jon wished for a $100 bill, for example, then $100 would be charged to an appropriate line of Credit. If Jon wished he were a gorgeous, curvy, oblivious MILF, then surely his mother would turn into an awkward, horny teenage boy. If Karyn wished she were a Magical Girl Championing the Forces of Goodness and Love and other cute fluffy stuff like that, then maybe Zoe would become a Supervillain, Championing the Forces of Evil and Hate and other cool metal stuff like that. They would also both lose access to the Stone's Magic and have to rely on their own. Use your Imagination! But make your Imagination Fair and Balanced.
Oh, one more thing: trying to wish away this rule, or other rules on the 0SS, will just get you another Altered Stone subject to different rules. The number of Rules on the Stone is also a Quantity to Conserve.
Have fun!
