It's sometimes known as the Aquamarine Pineflower Tiara. It was given to the Queen Mother by George VI in 1948 for their silver anniversary. She then gifted it to her granddaughter Princess Anne in 1973 as a wedding gift.
The pinecones are made of diamonds and pear-cut aquamarines, and the cones are interspersed with large emerald-cut aquamarines.
Princess Anne wore it a lot in her younger years. In the montage below, the Queen Mother is wearing it on the left, and a young Princess Anne is pictured in it in the middle: