Beth- I am convinced that this is the same tiara worn by Queen Victoria in the image you supplied and the cigarette card of Princess Christian/Helena. The tiara was also worn by Princess Marie Louise (Helena's daughter) later in life. I can't find an image at the moment but your post has got me thinking- the photo of Marie Louise I have always assumed was a pearl tiara, maybe the turquoises were replaced? Maybe they are opals? This would tie in with your theory re: the necklace being opals?