They didn't carve it into anything because devout Muslims are prohibited from making representational art (they take the "no graven images" notion very seriously). They believe that representational art is a blasphemous parody of what God has already created. While I'm no big fan of any kind of religious zealotry, in this case it has led to the development of truly fantastic non-representational art, leading to what is some of the world's finest calligraphy, geometric art, and religious architecture.

In the case of the black stone, it is not something that a muslim worships; muslims worship only God, which has no image. (There are not even any representations of Muhammed, lest someone accidently get the wrong idea and start worshiping him.) The black stone is venerated because it came from and was blessed by God; to attempt to turn it into anything else would be impious in the extreme.

Cheers,

Nathan.