Introduction:
This scenario involves a situation where A finds a costly ring and sells it to B without knowing the true owner. Now, the question is whether the true owner can recover the ring.

Finder's Rights:
The general rule is that the finder of lost property has a right to possess and keep the property against the whole world except the true owner. However, the finder's right is not absolute and can be defeated by the true owner's claim.

Reasonable Efforts:
In this case, A made reasonable efforts to discover the owner before selling the ring to B. This means that A may have posted advertisements, contacted the police, or made other efforts to locate the true owner. If A can prove that he made reasonable efforts to discover the owner, his finder's right may be strengthened.

Bona Fide Purchaser:
B, who bought the ring from A, is known as a bona fide purchaser. A bona fide purchaser is a person who buys property without knowledge that the seller's title is defective. In this case, B bought the ring from A without knowing that A was merely a finder. Therefore, B may have a good title to the ring.

Recovery:
If the true owner can prove that the ring was stolen from him or her, then he or she may have a right to recover the ring from B. However, if the true owner cannot prove theft and A made reasonable efforts to discover the owner, then the true owner may have a difficult time recovering the ring from B.

Conclusion:
In conclusion, the true owner may have a right to recover the ring if he or she can prove theft or that A did not make reasonable efforts to discover the owner. However, if A made reasonable efforts and B is a bona fide purchaser, then the true owner may have a difficult time recovering the ring.