Oak marble gall wasp

I thought this was a brown acorn left on the tree but was pleased to do a double take and discover something new.  I think it's an oak marble gall wasp nest (but could be wrong on that) 

If it is, apparently, it's the chemical that the wasp grub secretes that changes the way the tree grows to create the nest around the little wasp rather than the wasp building it. And there's all sorts of different growth types from different wasps including one that looks like an artichoke.

I was surprised to find the adult wasp is only 2mm long. Unsurprisingly, you see the galls more often than the wasp!