It's in His Kiss: The 2nd Epilogue (Bridgertons 7.5)

What happened to the diamonds?

If you've read "It's in His Kiss, " you want to know. Don't miss this charming and passionate addendum, in which Gareth proves that some things get better with age, Hyacinth gains new respect for her mother (with a daughter like Isabella she'd have to, wouldn't she?), and everyone learns that while diamonds are lovely, sometimes tenacity is a girl's best friend.