Two stars have recently been identified by astronomers using the Chandra as being possible quark stars. These have so far just been a theory: that the extreme pressure in neutron stars could break the neutrons down into their constituent quarks.

See also:

• Is RX J185635-375 a Quark Star?
• New Constraints on Neutron Star Cooling from Chandra Observations of 3C58

Hear also: This NPR story on the discovery, which is where I first heard of it.