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Power Gem is a rather interesting move, as it is one of the two only special Rock-type moves in the game, the other being AncientPower. Unfortunately, the Pokemon who have access to this move usually can make better use of physical Rock-type moves, mostly due to their higher Attack stat in comparison to their Special Attack stat. Thus, Power Gem is a relatively useless move when there are better, more powerful Rock-type moves available that can be used to great effect by physically oriented Pokemon, such as Stone Edge and Rock Slide. The main exception to this being Probopass, who sports a higher Special Attack stat.
Ancientpower is an option with slightly less power and much less PP that has a 10% chance of increasing all of the user's stats by one stage.