What sparks Arthur Donnithorne's suspicions regarding the relationship between Hetty and Adam in the novel, Adam Bede?

A fancy locket with a gemstone given to Hetty Sorrel by Arthur Donnithorne is what betrays her affair to Adam Bede. At a harvest celebration dance, Hetty's locket comes apart and drops to the floor. In helping her to pick up the pieces, Adam asks himself who gave it to her since Hetty clearly could not afford to buy one herself.