This tiara was made by Cartier Paris as a wedding gift for Princess Charlotte, mother of Prince Rainier, grandmother of Prince Albert. It was a gift from her husband, Count Pierre de Polignac (Prince Pierre of Monaco, Duke of Valentinois, as he became).
(Yes, that is another tiara around her neck)
The tiara is made of platinum and white gold, and features diamond scroll work with hanging pear-shaped pearl drops, which is why I'm choosing to call it the Cartier Pearl Drop Tiara.There are no pictures (well, as far as I know, at least) of Princess Charlotte wearing this piece. There are also no pictures of Princess Grace wearing it, so we jump to Caroline. Because Grace never wore the tiara publicly, many have assumed it passed directly to Caroline. Princess Charlotte was said to not have approved of Grace, so perhaps she did not leave her jewels to her daughter-in-law. I have no idea if Charlotte's will was ever public, which would be the only way to know for sure. And I think you have to consider that Charlotte died in 1977, and Grace died in 1982 - not giving Grace many years to wear it, if indeed the tiara did pass to her to stay in the reigning line.