Cynips quercusfolii is a small black gall wasp.  Agamic galls are cherry galls on the leaves of some species of oak. The galls are large and succulent, 15 to 25 mm in diameter, smoother on Quercus robur or more warty on Quercus petraea; yellow-green suffused with pink or red, and later dark red then brown with a thick walled chamber. They persist on fallen leaves over winter.

Provide photographs of the gall on the host plant, and note the host species.

English/Sessile oaks.

See British Plant Galls (Redfern & Shirley 2023).