This flower brooch is in the form of a spray of millet (sometimes called sorghum), a crop that is abundant in Botswana. It features 11 pear shaped diamonds to create the heads of the millet, with simple gold for the stem and leaves.
The President of Botswana, Festus Mogae, presented the piece to The Queen in 2007, at the Commonwealth Heads of Government Meeting. The brooch wasn’t exhibited until 2009 though, says From Her Majesty’s Jewel Vault. You can read more about that here.