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Did you give her the ring as part of any other gift-giving event, like a birthday or Christmas? If so, most jurisdictions wouldn't require her to give it back. But if it was just an engagement ring, not doubled up with any other occasion, most courts would have her return the ring because it was a "gift given in contemplation of marriage." That is the general rule.
(Although common courtesies in our culture might say if you broke it off, she doesn't have to give it back. If she breaks it off, she does.)
If you sued, you would likely prevail. And if the ring is worth $20k, it's worth it to pursue it.
Where did you both live when you got engaged? That may control where the more appropriate venue is--Oregon or Washington.
I'm not licensed in Oregon and don't know the exact specifics there, but I did find this post that puts a lot of info together, including some Oregon-specific stuff: http://blog.oregonlegalresearch.com/2009/09/the-law-of-engagement-rings-with-a-side-of-ring-flingers-and-oregon-cows.html
This site covers the basic details I ran over previously, but as you can see, certain facts and details can change the outcome.