From Worms Knowledge Base
The Stone works like a grenade, in that it is thrown, is affected by gravity but not wind, and bounces around the landscape. But unlike a grenade, the Stone does not explode. It only damages a worm if it hits that worm directly, and the damage is not fixed, but depends on how fast the stone is travelling when it hits the worm.
The Stone can continue to hit worms and damage them while bouncing around. when it comes to a rest, it does not disappear, but remains on the landscape like a dud mine. Like a mine it stays as a solid object, unlike a grave which worms can pass straight through. The Stone can be shot at, but not destroyed; it simply continues to bounce around the landscape causing trouble.
What's more, the Stone comes in five different sizes, which the player can alter using the numbers 1 through 5. Larger stones are bigger, and cause much more damage on impact, but cannot be thrown as far or fast, and don't bounce as well as smaller stones.
The only problem that might come from programming this weapon is in calculating the amount of damage that should be done to the worm depending on the speed and size of the stone, a problem that the Bowling Ball would also have to face.
This weapon provides many new opportunities for strategy and skill. A player would have to be extremely accurate with this weapon to cause damage, or he could use gravity to his benefit by dropping the stone from high above.
There is the slight drawback of the variations in size of the Stone possibly being abused in very cheap shots. If a player has his worm standing right next to the enemy worm, he is going to choose the biggest stone and throw it as hard as he can. But since larger stones cannot be thrown as fast, then it's possible to ensure that a small stone does just as much damage as a big stone. It's still a cheap shot, but at least the player gains no benefit from it being a bigger stone.
Another small strategy is in using the stone to block a worm in a tunnel or a tight space, requiring the enemy to shoot it out of the way to escape. This could also be used for cheap shots.
Such a simple object with so many uses, this weapon would make an excellent addition. And the comical value of stones being thrown around by explosions like shrapnel and hitting worms is too great to ignore.
Perhaps stones could also be present on the terrain when games begin, just to cause trouble.