Density Control

Density Control is the ability to manipulate the density of an object or ones' self. A character with such an ability can make the density of an object or their own body so low, that they can become intangible, or they can make the density so high, that the object or person would become as hard or harder than diamond.

The Martian Manhunter for example, has this ability, but it seems to be limited to the alteration of his own density, as he cannot alter the density of some else or something else.