Hazelnuts or Cobnuts, fruit of Corylus avellana.
Image: 'Nova'
Cobnuts could be used for a game similar to marbles. 'Playing Cobnuts' is mentioned in John Boyle's 1752 Life of Jonathan Swift, and in this from 'The Mill on the Floss' by George Eliot:
" "No," said Maggie. "...Tom! Is it marls (marbles) or cobnuts?" Maggie's heart sank a little, because Tom always said it was "no good" playing with her at those games, she played so badly."
"Marls! no; I've swopped all my marls with the little fellows, and cobnuts are no fun, you silly, only when the nuts are green..."
Kentish Cobnut Association