回複：請問我可以要回訂婚鑽戒嗎？

From a pure legal point of view, the $30,000 or the engagement ring was a consideration in exchange for marriage or promise to marry. Thus, Statute of Frauds will govern. Which means, unless you have a written record the chance that you can get the ring back through legal system is minimal, if not zero.

From a moral point of voew, your girl friend should return the ring to you if she decided not to marry. That's a common decency.