I was finally able to finally check out the Cartier exhibit at the Denver Art Museum: Brilliant: Cartier in the 20th Century. It runs until mid March and if you are in the area (or in need of a ski trip) I highly recommend it. It seems like an extension of the traveling exhibit they've had a few other places, but with some additional items too. A handful of marked royal items, with a few others that I think were marked "Private Owner" but had royal connections.
Princess Grace's engagement ring is a stunner as were the handful of tiaras they had - they sure do sparkle in person (more than I imagined.)