This sparkly delight was made in 1976 by Van Cleef & Arpels and is composed of white gold, platinum, and 144 diamonds totaling 77.34 carats. Princess Grace wore it once, for the pre-wedding ball celebrating Princess Caroline's first marriage to Philippe Junot in 1978.
It certainly does have a lot of sparkle, and a larger size than I would guess from the solo pictures. You can see it in action in this video, starting around 4:05 (caution: nerdy Albert ahead):
Though it was worn by Grace, it is to this day still owned by Van Cleef & Arpels. These days, the tiara tours with exhibits. It's been shown with the Grace Kelly Years exhibit, and is now on display at the Cooper-Hewitt Museum in New York City with the Set in Style: The Jewelry of Van Cleef & Arpels exhibit (through July 4, 2011).
Its most recent star turn? In the 2012 movie Mirror Mirror, where it was worn by Lily Collins and Julia Roberts.