All it says is a diadem belonging to Queen Adelaide. There hasn't been any information about this diadem, other than references to it in relation to Queen Victoria's Regal Circlet, which really just further confused matters. There is no information about what this diadem looked like, when it was made and what happened to it and the stones.
Since it has been clearly mentioned here, and there is a whole chapter dedicated to Queen Adelaide in the new publication. I don't think I am being unrealistic in hoping that the diadem will be covered in some detail.