Such as on this piece of which there are only 10 known that were found by a World-famous antiques dealer in Paris. I bought all 10 of them 30 years ago and sold, traded and gifted the best of the lot... One example is in the NICE book and none have been reported found, since.

I'd like to trade this piece for a nacre from Nice, France, or maybe anywhere in France; in the 400-500 range.

Note the blemish in the bottom photo.