It is a sapphire. The sapphire used could be used in the honeysuckle tiara and in the wing tiara.
Princess Margaret wore it in the wing tiara (as well as Queen Mary) and as a brooch.
Queen Mary wearing it as a brooch in 1911. This photo was found by Russian Treasure on Instagram https://www.instagram.com/russian_treasure/ and given to British Royal Jewels on Instagram see https://www.instagram.com/britishroyaljewels/ who posted about it at some time.
Russian Treasure's find was very significant as it proved beyond doubt that the sapphire had not been purchased from the estate of Empress Marie as many had previously argued.
Princess Margaret wearing it in the wing tiara
As a brooch