What is the difference between a crown and a diadem?

"Diadem" literally means "crown", but can also mean the jewels and decorations on a crown. According to this site, diadem and crown are synonyms.

https://www.vocabulary.com/dictionary/diadem

A diadem is a specific type of crown, more like a headband in appearance (and originally made of cloth, not metal), but still signifying the royal authority of its wearer. Think of the paper crown that comes in a Christmas cracker, a loop of tissue paper with points rising up from it, to get the general idea of the diadem. However, it must be acknowledged that some items designated as crowns are in fact shaped like diadems. Some of these distinctions of terminology are very specific to the United Kingdom.

https://en.wikipedia.org/wiki/Diadem