You are correct; the diamond ring is now an asset of the marriage.
While a couple is still engaged, the engagement ring is considered a conditional gift and therefore the fulfillment of the gift is conditioned on the marriage taking place. If the parties end their engagement and do not get married, the ring should be returned to the purchaser.
However, once the parties become married, then the ring – like every other asset that the parties own – becomes a joint asset of the marriage.
Certified Divorce Financial Analyst
Business Valuators / CPAs