Hugh Hefner's ex-fiancee Crystal Harris ... just put her engagement ring on a pretty famous auction

A diamond isn't always forever ... sometimes it's only for 6 months -- especially for Hugh Hefner's ex-fiancee Crystal Harris ... who just put her engagement ring on a pretty famous auction block!

TMZ has learned ... the platinum ring, which boasts a 3.39 carat circular-cut diamond -- will be auctioned off by Christie's in NYC next month.

According to the listing, the ring is expected to sell somewhere in the range of $20 -$30k.

Sources close to Crystal tell us ... the former Playboy model "couldn't bear to look at the ring anymore because it brought back bad memories."

I call BullS#$%