This is the Lennox Jewel (or Darnley Jewel), said to have been commissioned by Lady Margaret Douglas for her husband Matthew Stewart Earl of Lennox. It may have been a memorial piece to mark his death in battle in 1571, or possibly it was made to mark his return to Scotland in 1564 or the return of his lands and honours in 1565. The piece was bought by Queen Victoria in 1843 and the Royal Collection Trust currently have it on display in Mary Queen of Scots' Outer Chamber at the Palace of Holyroodhouse in Edinburgh.

Margaret Douglas was Margaret Tudor's only surviving daughter – I described her traumatic birth and difficult childhood in Three Sisters, Three Queens, and her wedding to the Earl of Lennox in The Taming of the Queen. In The Last Tudor, the book I'm working on at the moment, she is one of Elizabeth I's potential heirs, a rival to the heroines the Grey sisters.

https://www.royalcollection.org.uk/collection/28181/the-darnley-jewel-or-lennox-jewel

Images: The Darnley Jewel or Lennox Jewel, c.1571–8, Royal Collection Trust/© Her Majesty Queen Elizabeth II 2017, RCIN 28181; Lady Margaret Douglas, Countess of Lennox (1515–78), c.1572, Royal Collection Trust/© Her Majesty Queen Elizabeth II 2017, RCIN 401183; Margaret Tudor, Queen of Scotland (1489–1541) by Daniel Mytens, c.1620–38, Royal Collection Trust/© Her Majesty Queen Elizabeth II 2015, RCIN 401181