My fiance broke up with me but hasn't asked for ring back. Advice please!

But usually it is the party who did not break the engagement who is entitled to retain it.

http://www.legalmatch.com/law-library/article/return-of-the-engagement-ring.html

Ring as a Promise

Unfortunately, a majority of courts find that the gift of an engagement ring contains an implied condition of marriage. That means that until the marriage occurs, the gift isn't final. The courts that view giving of engagement rings this way indicate that only upon the exchange of wedding vows does the donee get to keep the ring. Up until that point the ring is only the donee's on the condition of marriage.

Courts that follow this line of reasoning treat the ring more as a term in an oral contract than as a true gift: the ring becomes the donee’s property on the condition that the marriage is completed. The ring is thus a promise that the donee will marry the donor. If the promise is broken by either party, than the ring remains the property of the donor, regardless of the reasons the promise was broken.

Ring as a Gift

Personal property is considered a gift when three conditions are met:

The donor intends to give the property as a gift.
The donor gives the property to the donee as a gift.
The donee receives and accepts the gift.
A few states believe that once the donor gives the ring to the donee, the ring is a gift and the ring becomes the donee’s property, regardless of what happens later. These states, known as no-fault based states, use the same logic found in no-fault divorce; they do not care who was responsible for the end of the engagement and believe the ring always belongs to the donee after the ring is given.

Ring as an Implied Gift

Some courts follow a middle ground between the two extreme positions by determining which party broke the engagement. These states, known as fault-based states, find it unfair to the donee if the donor breaks off the engagement because the donee was preparing themselves for marriage. The national majority approach is that a donor who breaks off the engagement for a reason that has nothing to do with the donee's behavior cannot recover the ring. Other courts find it unfair for the donee to keep the ring if the donee's wrongdoing or unfaithfulness was the cause of the break up.

What Other Factors Might Influence the Court’s Decision?
Although engagement ring disputes are typically based on fault or no fault, there are a few other factors which may sway a court’s decision:

Family heirloom- If the ring originally belonged in the donor’s family, the donee should return it as the donee no longer intends to join the donee’s family with the donor’s family.
Special occasion – If the ring was given during a special occasion, such as Christmas or a birthday, the ring may be treated like any other gift: the ring belongs to the receiver and has no obligation to return it.
Prenuptial agreements – Prenuptial agreements are contracts made about the division of property before the couple is officially wed. Since the prenuptial agreement is actually a written contract, the agreement will supersede, or replace, the assumptions made from any oral agreements.