The Strathmore Rose tiara was given to Elizabeth Bowes-Lyon (who later became George VI's Queen Consort, and then the Queen Mother) by her father, the Earl of Strathmore, as a wedding gift in 1923.
It was a delicate floral tiara, with five roses made of diamonds, separated by diamond sprays of leaves. It was meant to look like a garland of wild roses.
It could be worn as a bandeau across the forehead (the fashionable style of the 1920's), or as a tiara on the head. The diamond roses could be removed and worn as brooches.
The Queen Mother wore it a lot in the 1920's, but put it away as she got older in favour of more substantial tiaras. It hadn't been seen for a century till Catherine, Princess of Wales wore it to a state banquet this week.