megan fox pink dress brian austin green black suit gq

Megan Fox and Brian Austin Green are engaged... Again. Photo: Steve Granitz,

He popped the question... and then the ring popped off her finger.

Actor Brian Austin Green proposed to girlfriend Megan Fox on a Hawaiian beach, reports Us Weekly, but the ring somehow slipped off the sexy star's dainty digit and remains missing.

Green and Fox, who were previously engaged in 2009, were vacationing at the Four Seasons Hualalai hotel on the Big Island in Hawaii when he reportedly presented her with a 2-carat dazzler that they had picked out at Excalibur Estate Jewelry in Beverly Hills, Calif.

The jewelry store told StyleList that the stunner -- estimated by Diamond Information Center expert Francesca Ferrante to be worth $60,000 -- is a platinum ring from the 1930s featuring an old mine cut with filigree sides and pavé diamonds.

(Platinum has become popular among affianced celebs like Carrie Underwood and Nicole Richie because it enhances the brilliance of the diamond.)

An onlooker who witnessed the Emporio Armani Underwear model's beachside proposal -- and subsequent ring loss -- told Us Weekly:

megan fox platinum engangement ring 1930s old mine cut filigree sides and pavé diamonds

A similar ring to the one Megan Fox and Brian Austin Green lost in the sand. Photo courtesy of Excalibur Estate Jewelry

"I saw her jumping up and down. Later, I saw a half dozen staff sifting through the sand."

They might have found a few stray cigarette butts, but the ring never turned up.

A source told the magazine, "Security and maintenance staff spent a couple of hours looking for it. No one found it."

Very sad, especially considering that the "Jonah Hex" beauty could probably also use her too-big ring as a belt for her 18-inch waist.

To see how Megan Fox's sparkler stacks up in the bling arena, check out our celebrity engagement ring gallery.

And in related news, read about the latest trend in engagement rings.