A type of head ornament worn by ladies, usually royalty or members of the nobility, on state or formal occasion. Tiaras vary greatly in form and degree of ornamentation but usually are curved (less than a semi-circular) vertical band with a central peak, all encrusted with diamonds or other gemstones.
In the Victorian era in England some necklaces were made so that the central part could be detached and worn as a tiara; in Russia and England a type of necklace could in its entirety be worn as a tiara. The term originally was applied to the head-dress worn by the ancient Persians; it is now also applied to the triple crown of the Pope.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson