A tiara is a jeweled head ornament. Its origins date back to ancient Greece and Rome. In the late 18th century, the tiara came into fashion in Europe as a prestigious piece of jewelry to be worn by women on formal occasions. The basic shape of the modern tiara is a (semi-)circle, usually made of silver, gold, or platinum, and richly decorated with precious stones, pearls, or cameos. Tiaras were extremely popular during the late 19th century and were worn at events where the dress code was white tie. After World War I, wearing a tiara gradually fell out of fashion, except for official occasions at a royal court. Interest in tiaras has increased again since the beginning of the 21st century. The word "tiara" is often used interchangeably with the word "diadem".

Source: Wikipedia