What is it?

We wondered as I spotted these brown spheres on a little tree as we walked. Not seen it before, but the fact that it was oak gave me the clue - gall - home to oak gall wasps. Probably oak marble gall. 


The RHS says, "Oak gall wasps are part of the biodiversity that a healthy oak tree supports. They little or no impact on the tree's health and growth is minimal, therefore to attempt to control them is undesirable."

Galls are rich in resins and tannic acid and have been used widely in the manufacturing of permanent inks (such as iron gall ink) and astringent ointments, in dyeing, and in leather tanning.

The biblical term gall (such as offered to Jesus on the cross) typically refers to a bitter-tasting substance made of a plant such as a wormwood or myrrh.