I have been told that diadema is translated as a crown representing royalty and/or authority. I see in Rev.19:21 the crown of Christ is diadema. Rev 6:2, the one on the white horse bent on conquest (antichrist) wears a stephanos. Now, I read in Rev. 13:1 that the beast with ten horns and seven heads is wearing a crown (diadema). Also, Mark 15:17 uses stephanos. I thought I had a handle on the difference until this. All the Greek dictionaries I have access to translate those as the same definition with the exception that stephanos is twisted or entwined together.
διάδημα versus στέφανος
διάδημα, as defined by the Perseus Digital Library's entries from LSJ and Middle Liddel, is a rather straightforward 'band' around the head. The LSJ entry provides examples that quickly associate a διάδημα with royal authority, a type of crown worn by kings.
στέφανος has a much longer set of definitions, but the one that Revelation is most concerned with would be that a στέφανος was a 'crown of victory' or 'crown of honor'.
διάδημα is used just three times in Revelation: worn by the dragon in 12.3, the beast in 13.1, and the rider of the white horse in 19.12.
All three instances are associated with royal authority in some way: the dragon is said to have 'power', a 'throne', and 'authority' (13.2); the beast receives the dragon's throne and authority, and is further said to have 'kings' (17.10); and the rider of the white horse is called 'King of kings and Lord of lords', and is described with language taken from Psalm 2, a song about Israel's king (19.15-16).
στέφανος is used eight times in Revelation: a reward for martyrs in 2.10, a reward for perseverance in 3.11, worn by heaven's elders in 4.4,10, the rider of the white horse in 6.2,1 the woman in 12.1, and the son of man in 14.14; the final instance is 9.7, where it appears in simile ('on their heads were what looked like crowns of gold').
Three of these seven instances appear in the immediate proximity of the verb νικάω, 'to conquer' (2.10-11; 3.11-12; 6.2). Such 'conquest' is implied for a fourth instance (the στέφανος in 14.14 is worn by the son of man, who is identified as Jesus in 1.7,12-18, who is said to have 'conquered' in 3.21 and 5.5).
Within Revelation, John reserves the διάδημα to depict royal authority (good or bad), while he tends to use the στέφανος to indicate the victory of conquest (good or bad).
1 Just as an aside, because it came up in the course of the original question: While the NIV renders Revelation 6.2 as 'he rode out as a conqueror bent on conquest', this is not an accurate reflection of the Greek, and shows a significant bias on the part of the translators in designing the verse to come across as immensely negative, and therefore lead readers to view the horseman as 'the antichrist'. The more neutral 'he came out conquering and to conquer', found in many other translations, is a more literal translation of the Greek. See, e.g., this answer elsewhere on Hermeneutics, for why the 'conquest' of this horseman may have been meant by John as a good thing, not an evil thing.