The agamic (asexual) galls are rounded, 3 to 4 mm, often in clusters and arising from a dormant bud on the trunk  or twigs of oak trees, often in callus tissue. The surface has rings of come like bumps and may be finely pubescent. It is soft and greenish-yellow at first, later becoming red then brown and woody.

[Note: The sexual galls occur in buds of Turkey Oak, but none have been found in Britain as yet.]

Photograph the gall on oak, and note the host species

native oak trees

The gall was not found in Britain until 2008 

uncommon, but may be increasing

See British Plant Galls (Redfern & Shirley, 2023)