Gems and Jewels by Egantx7

Question 7

This diamond was cut from a larger diamond in hte French crown jewels, sometime after Louis XVI's execution. It was later made into a necklace, and purchased by Caledon Hockley. While a copy, with a platinum set 171 carat sapphire instead of an actual diamond, was created by Asprey & Garrard and auctioned for Princess Diana's charity, the more famous (and less valuable) original is owned by J. Peterman. Identify this impressive looking stone that no one is looking at when it appears around the neck of a woman being sketched.

Coueur de le Mer (Heart of the Ocean)