Tiaras used to be de rigeur among a certain social class. To make them more useful, they were often convertible into necklaces and some broke apart (depending how ornate they were) into additional pieces.

This gorgeous vintage aquamarine and diamond tiara from Burlington Arcade jeweller Michael Rose can also be worn as a necklace, as shown (top), and bottom, on its frame as a tiara. It is available for £17950.