At the begining of each upkeep, put a gem counter on gem grab if their are more than 2 players. Otherwise, put 3 gem counters on it. Then, If creatures that player controls have 10 or more total gems, exile gem grab and they gain 8 trophies.

When a creature dies, put its gem counters onto gem grab.

Whenever a player is dealt combat damage by a creature, move all gem counters from gem grab to that creature