Offering a son a diamond to use in an engagement ring

I think the ring should definitely be discussed ahead of time. Although my diamond came from my grandmother’s ring, I picked out my setting. Good friend of ours were together 5+ years before they were engaged. Although he had a great paying job, he chose a very dimple ring with a small diamond. About a month later I noticed she was wearing a much larger ring. There are just so many different opinions not to have a conversation.