In Delaware, the donor of a ring is entitled to its return where the engagement is mutually broken.  In Machurek v. Wilson, the Delaware court opined that the rationale for this rule is that an engagement ring is a gift conditional on the subsequent marriage of the parties, and when the condition is not fulfilled, the donee no longer has any right to the ring.[1]

[1] Machurek v. Wilson, 2007 Del. C.P. LEXIS 40 (Del. C.P. 2007)