Bristol Palin wears a rock -- and rocks her mom's world -- as she announces her surprise engagement on the cover of the new Us Weekly.

"We got engaged two weeks ago," reveals 19-year-old Palin, who wears a white A.L.C. frock as she flashes the 1-carat, square-cut diamond engagement ring given to her by on-again flame Levi Johnston.

Twenty-year-old Johnston, in a white Calvin Klein shirt, is seen embracing his fiancée, who holds their 18-month-old son Tripp.

Jewelry and style expert Michael O'Connor tells StyleList that Palin's sparkler, estimated to be worth $8,000, looks to be set in platinum on a pavé band.

"The pave diamonds make it a modern-style ring," O'Connor says, "and since the diamonds are set in platinum, the stones appear larger."

One person who hasn't yet seen the ring is former Alaskan governor and vice-presidential candidate Sarah Palin, who had not been told of her daughter's engagement prior to Us Weekly hitting the newsstands.

Uh, surprise!

This morning, Sarah Palin and her husband, Todd, issued a statement, which was read on the "Today" show: "Bristol at 19 is now a young adult. We obviously want what's best for our children. Bristol believes in redemption and forgiveness to a degree most of us struggle to put in practice in our daily lives."

The young Palin, who has promoted abstinence in Candie's PSAs and discussed the joys and struggles of single motherhood in Harper's Bazaar, was initially engaged to Johnston in May 2008, when she was pregnant with Tripp. Two weeks after the baby's December 27, 2008 arrival, the lovebirds split.

Despite a messy breakup -- after which Johnston trashed the Palin family and bared his butt in Playgirl -- the high-school sweethearts reconciled three months ago when they met to discuss a custody arrangement for their son.

"I thought about being with her all the time," says Johnston, who fondly recalls a walk during which Palin "made fun of my straight-legged jeans and my hair." But not the Playgirl shoot? Come on!

After that, it was on, as they say. "We started hanging out," explains Palin, who models an Alice + Olivia dress and Emitations bracelet inside the magazine. "And literally, we have hung out every day since."

While Johnston's first proposal was somewhat informal ("We were spooning and he just slipped a ring on my finger. Didn't even ask me," Palin remembers, adding that she demanded a different ring because her friend had the same one), this time, Johnston was much more romantic.

Palin, a medical assistant at a dermatologist's office, recalls, "I came home from work. There were rose petals in the shape of a heart all over my bed, with a box in the middle.... Then he got down on one knee."

The couple plan to marry "soon" and dream of an outdoor wedding in Alaska. "It will be a Christian ceremony," explains Palin, who intends to stay celibate until the big day.

"And we'll definitely incorporate camouflage. I always wanted Levi to wear a camouflage vest. Tripp can have a vest, too. He'll be the ring bearer."

While Johnston -- who also wears a J.Crew shirt with Brooks Brothers khakis in the Us Weekly photo shoot -- hopes to gussy himself up in a Hugo Boss suit, Palin plans to don a white version of the $3,400 red Carolina Herrera dress she modeled in Harper's Bazaar last month.

Most important, she tells the magazine, "I've always wanted a milk-chocolate fountain! We just want it to be fun."

Just don't get any chocolate on that dress!

In related news, we wish Bristol Palin would wear this to her wedding.