Okay, I've been trying to dig into this for ages. Time to draw upon your experience. The Hellenistic 'diadema' as worn by Alexander the Great and his diadochi descended successors - do we know what material it was made from? Most of the ancient portraits I have examined suggest a fabric, but beyond that I cannot discover. I would also like to know what colour was the diadema - some papers say gold (thread, I presume), some say white, some say purple or a mixture of purple and white. In addition, I've seen quite a lot of museums artefacts labelled as diadems, but they are actually made of metal (usually gold) and clearly intended to be seated on the head more like a modern tiara - are these 'diadema' too or are they just confusingly labelled? I have also seen that diadochi queens sometimes wear the stephane instead of the diadema, but have also seen the stephane in depictions of goddesses and in portraits of non-royal women. Was there a protocol about who could wear the stephane and for what occasions, and also for the diadema (which does seem to be restricted to royalty)? Some specific individuals seem to never wear the stephane, or vice versa the diadema, despite being queens - is that simply down to fashion trends? Finally, a question about the radiate crown - what was it made of, and again were there any specific protocols governing its use? Visually it seems to have evolved out of the fabric diadema but I would assume it is made out of metal, and I have only seen it on a specific few kings, with others never depicted wearing it.
/u/cleopatra_philopator - you seem like you might be able to provide some much needed insight!
The noun διάδημα is derived from the verb διαδέω, "to bind around." So literally it just means "something bound around, a headband." But as Smith in Hellenistic Royal Portraits notes, "the noun diadema, which also looks strangely non-specific [as is the verb], had from its beginning the limited sense of "royal headband." Smith mentions that modern interpreters frequently mistake any Macedonian headband for a diadem, pointing specifically to a metal headband found at Vergina that was erroneously identified as a diadem. Smith distinguishes between two types of diadems found in iconography, the so-called "flat diadem" and the "rolled diadem." Both types appear almost exclusively to be made of cloth (the presence of tassels generally seems to affirm that), although Smith says of the flat diadem that "the flat band may be completely plain or can have raised edges, which may have been of gold." The rolled diadem Smith says is in origin apparently just the flat diadem twisted into a tubular form, but he also notes the existence of some tubular metal diadems of this sort: "A headband of this type, made either of rolled cloth or tubular metal, is worn by a few certain, probable, and possible royal portraits, where its precise significance, if any, is not clear."