Can something denser and harder than osmium exist?

[Swami]

I have a question about density: would it, in principle, be possible to create something with greater density and hardness than, say, Osmium? Or is that impossible? Is density constrained by the elements of the periodic table, or can something with greater density be created? The finished product should be stable and usable, like a metal -- so naturally a miniature black hole or portion of a neutron star wouldn't really work.

Any thoughts?

 

[rumborak]

Weelll, given how natural Osmium has a rather widespread distribution of isotopes, you could try to purify one of the higher isotopes, thus achieving a higher density.
Probably not what you had in mind though :)