IN-DEPTH FEATURES & LISTS

Occasionally, between current events and posts on single jewels, we dig a little deeper on a certain topic with a feature or an index, a listing of jewels that share a common characteristic. 
What we've covered so far:

The Hon. Mrs. Ronald Greville bequeathed her jewels to Queen Elizabeth in 1942. Here's who she was...

...and here's her jewelry collection, including an ongoing list of jewels known to have been in the bequest.
India in The Queen's jewel collection, inspired by The Duke and Duchess of Cambridge's first royal tour of the country.

Index: The Queen's Bejeweled Birthday Presents
What do you get the woman that has everything for her birthday?