What is does Chillingworth finds on Dimmesdale's chest at the end of chapter 10 in "The Scarlet Letter"?

At the very end of chapter 10 of "The Scarlet Letter" by Nathaniel Hawthorne, Chillingworth finds something on Dimmesdale's chest but the narrator doesn't say what it is. I know it says it later in the book, but being very bad at interpretting Old English language, I can't figure out what it is. What is it?