In the reign of King Rama IV, a set of diamonds was added to the crown. Also added was a large cut diamond from India to decorate the top of the crown, called the Great Diamond (พระมหาวิเชียรมณี Phra Maha Wichian Mani). The crown is of a distinctive Thai design, being a multi-tiered conical diadem, terminating in a tapering spire.
The crown is worn only when a king is crowned. He places the crown on his own head. The shape of the crown represents the concept of divine monarchy. The tall spire signifies divine authority and the right to rule over his subjects.
The Great Crown of Victory is the most important of the five regalia of Thailand, yet at one time the crown was accorded the same importance as the other regalia.
According to wikipedia