Stone

Kirby's transformations into various Stone items

Stone is Kirby's Down B special move. Kirby transforms into a rock, becoming impervious to damage for a short time. Kirby can still be taken out of Stone form by grabs or grab-type moves such as Yoshi's Egg Lay or Lucario's Force Palm. He can deal damage by using the move in mid air and falling on opponents.

Although he cannot be harmed while stone, he can still be grabbed, so it's best to end the transformation early by pressing the B button again.

In Super Smash Bros., Kirby can only transform into one type of stone.

In Super Smash Bros. Melee and Brawl, Kirby can transform into one of the following:

Regular Stone Kirby
A spiked ball
A 100 ton weight, similar to his 8 ton one from Kirby Super Star
A garbage block from Panel de Pon
A Thwomp from Super Mario 64

In Brawl, the move is more powerful than Melee, and it pushes characters away, even if used from the ground. However, very strong attacks and hazards are capable of knocking back Kirby out of his Stone form (it will take 30% to break him out of this form), such as the cars on the Onett stage, though he won't take any damage.

Origin

The move is one of Kirby's copy abilities from Kirby's Adventure, behaving in much the same way. Kirby Super Star introduced the ability for Kirby to transform into various other stone objects including a rarely seen golden statue of Mario or Samus. It seems that games before Kirby Super Star it doesn't slide on sloped areas. Games after and Kirby Super Star put in the sliding mechanic as Stone Kirby.

Trivia

In Brawl, it can be tested in Boss Battle Mode and SSE that Kirby never automatically turns back into his normal form while being in his Stone form in midair, or at least for a very long time. However, in normal Group Brawls, this will hardly be useful.
There is a glitch associated with this attack on the PictoChat stage, widely known as the Earthquake Glitch. If you go to the very edge on the right-hand side and use the Stone move, Kirby will create shockwaves that cause massive damage.
Kirby's stone attack can be used to avoid some final smashes, such as Ganondorf's, without taking any damage. However, he takes full knockback. This technique is unreliable against some characters Final Smashes, such as Marth's, due to the high knockback received from the attack.
The SSB stone looks similar to an item from Paper Mario:The Thousand Year Door