While recently looking for something completely different on Wikipedia, I came across the photo below of Gwenc'hlan Le Scouëzec, the Grand Druid of Brittany who died in 2008.
I was struck by how similar his headdress is to the Midnight tiara worn by Crown Princess Mary. I have not done much research into his organization, but it appears that the Grand Druid is the only person who wears this piece.
This is not mentioned as an inspiration by the tiara's designer Charlotte Lynggaard of Ole Lynggaard. Does anyone else see a striking resemblance or is this merely coincidence?