A diadem is an ornamental headdress that is designed to indicate royalty, high standing, or dignity. It has evolved significantly since it was first used among the Persians, and museums that specialize in items from classical antiquity often have a very interesting collection of depictions of these headdresses that illustrate the history of this noble headgear. While the diadem continues to be associated with royalty today, it is also possible to see ones on the heads of those who are definitely not associated with royalty, worn as decorative accents.
The word comes from the Greek diadein, which means “to bind around.” The earliest versions were bands of white fabric that were wrapped around the head and tied, classically with the ends trailing over the shoulders. Only royalty were entitled to wear them, and so they came to be viewed as a symbol of royalty. Over time, various cultures in the Mediterranean devised their own versions, with ornamental additions like multicolored cords or stylized representations of various cultural symbols, such as the asp in Egypt.
In Greece, they began to take the form of wreaths, with the royal dead especially wearing golden wreaths in their tombs. Their use in turn gave way to the use of a golden band, and not long after that, people started adding precious jewels, and a form of the crown began to emerge. While this headdress is technically a circlet, rather than an upright crown, it was clearly the inspiration for the crown, and the modern half-crown known as a tiara is sometimes referred to as a diadem in a reference to this.
Many depictions of diadems can be seen in works of art from Greek and Roman times. In addition to appearing on the heads of rulers, they were also granted to members of the nobility, and some people took them for themselves as they conquered new lands and peoples.
The distinct image of the diadem is very evocative for some people, which may explain why it shows up in poetry on a regular basis. Royalty around the world continue to wear forms of the headdress, especially at formal events, and some monarchs seem to prefer them over more ornate (and heavy) upright crowns. Female monarchs in particular may choose to wear them exclusively.