Knopper galls

Evidently these are knopper galls on acorns, which are abnormal growths caused by the Knopper Gall Wasp. We have 4 oak trees and 3 are "normal" but the fourth is shedding these galls almost constantly. The ground underneath crunches as I walk over it, and when I go to the car I almost have to wear armour! 

When I researched them on Google I noticed that they are for sale on eBay. Evidently people buy them to make a type of medieval ink. £9.17 for 55gm!! I must have a fortune sitting on the drive!