Vow of purity until marriage is a noble choice.
There is nothing wrong with symbolizing one's choice with a piece of jewelry, be it a ring or a pendant.
3 of my (girl) friends waited 'til marriage, and their husbands (fiancés at the time) completely respected their choices. All three couples are still happily married with children.
In a world where sex is cheap and devalued, I admire and respect girls/young women who make that choice. It is not easy, hence the respect.
Cheers.