Power Stone

Power Stone (パワーストーン) is Stone Man's special weapon. When used by Mega Man, three stones shoot out and spiral about. It can be helpful to take out enemies in hard to reach (or dangerous) spots, such as the Power Massler in the second area of Wily's Castle (near the boss, Circling Q9).

In Mega Man 2: The Power Fighters, the Power Stone launches a giant stone hand that rises from the ground and moves forward.

Capacity
In Mega Man 5 and Mega Man IV, each use of Power Stone only drains one weapon energy unit. With a full weapon energy gauge, it's possible to use Power Stone twenty-eight times in Mega Man 5 and 19 times in Mega Man IV.

Bosses weak against Power Stone

 * Mega Man 5: Charge Man, Dark Man 4
 * Mega Man IV: Charge Man, Ballade
 * Mega Man (Game Gear): Star Man
 * Mega Man 2: The Power Fighters: Elec Man

Other media
Mega Man uses the Power Stone in the Mega Man animated series.

Trivia

 * Coincidentally, Capcom has a fighting game series called "Power Stone".