In The Crown Season 4, Episode 3, titled "," the "proper piece" refers to the iconic engagement ring Lady Diana Spencer selects from a tray of options presented by the royal jeweler.
: Diana chooses the blue sapphire ring, which was actually a stock item for sale from the jeweler Garrard's rather than a unique royal heirloom. [S4E3] Fairytale
Key details about this "proper piece" in the episode include: In The Crown Season 4, Episode 3, titled