From 2 years ago: a pair of galls on an oak tree.
The brown, spherical one is a cola nut gall which was home to a single female of the asexual generation of the wasp Andricus lignicola = Andricus lignicolus (Hymenoptera: Cynipidae). The small hole in its surface is probably where the adult wasp emerged. The marble gall is similar to the cola nut gall but lacks the brown flakes on its surface.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-09-24.