The Tennessee courts have held that in the absence of a contrary expression of intent, it is logical that engagement rings should be considered conditional gifts given in contemplation of marriage.[1]  If marriage does not ensue, ownership of the ring never vests in the donee and the donor is entitled to the return of the ring.  Thus, the engagement ring goes back to the one who gave it, if the marriage does not occur.

[1] Crippen v. Campbell, 2007 Tenn. App. LEXIS 600 (Tenn. Ct. App. Sept. 24, 2007)