This is the crown of Constantine IX Monomachos. It is gold with cloisonné enamel. Made in the 11th century, it features Constantine with his second wife Zoe and her sister Theodora. They also feature two dancers on either sides of the women.
The crown can be found at the Hungarian National Museum, although the site is not search-able. The University of Puget Sound Art Dept has a picture of the crown that can be zoomed in on. There is a similar item in the Victoria & Albert Museum.
Height: 10.6 cm, Width: 5 cm and Depth: 0.2 cm. Mid 11th century.
The Monomachos Crown: Towards a Resolution by Timothy Dawson. Via Google Docs.
The Full Wikis: Crown Jewels.
Byzantine Art from The Pages of Dr. Rozmeri Basic.