Kokoshnik is the Russian word for "cockscombe". It was the tradition for Russian brides of the 18th and 19th century to wear a kokoshnik headress holding their veil in place on their wedding day. Tsarina Maria Federovna then copied the style with a diamond tiara version. And in the process set a new fashion trend across Europe: the kokoshnik tiara.
Tsarina Maria Federovna was the sister of Queen Alexandra of England (Edward VII's consort) and she was determined to have a kokoshnik of her own. And here it is:
From left to right, Queen Alexandra, Queen Elizabeth II and Queen Mary wearing it. The tiara has more than 400 diamonds in it, and can be removed from it's base to be worn as a necklace.