This is the Strathmore Rose Tiara. Lady Elizabeth Bowes-Lyon (the future Queen Elizabeth, the Queen Mother) received it as a gift from her father, the Earl of Strathmore, for her wedding in 1923. The piece itself is likely older than that; it may have already been an antique when the Earl purchased it. The tiara features a garland of wild roses in diamonds mounted in silver and gold. Suzy Menkes states in The Royal Jewels that the individual elements can be removed and worn separately.
September 29, 1921: The Queen Mother when she was Elizabeth Bowes-Lyon with her father, the Earl of Strathmore.