Hef's Ex-Fiancee -- My HUGE Engagement Ring is On the AUCTION BLOCK!

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."

Think how the guy who paid for that rock feels ... right, Hef?