Knopper Gall.

This would have been an Acorn but the Gall Wasp caused it to change.
A Cynipid  gall wasp that causes common and familiar Knopper galls to form on the acorns of various oaks. The gall is seen more often than the adult wasp.
The knoppers are the agamic (asexual) galls, and are a mass of ridged tissue with a large central cavity and small inner gall. 
The sexual galls are found in Turkey oak catkins, and are small (1.5mm - 2mm) and rounded/cone-shaped; green at first and then brown.  They are common but rarely recorded