gleaming: bright enough to catch the eye, and so also showy, fine, and expensive
The picture behind this word is a surface that throws the light back at you. Something counts as gleaming if you notice it because it shines: a star, water running clear over its bed, linen so white it looks lit from inside, the clothing of a messenger from heaven. The gleam is the point, not the color. That is why the New Testament can set this word beside "white" (λευκός) and beside "clean" (καθαρός) in the same breath without simply saying the same thing twice.
From there the word slides in a direction English keeps in two separate words. Clothes that shine are clothes that cost money, and clothes that cost money announce who deserves the good seat. So the one word that describes an angel's clothing in Acts also describes the visitor's outfit in James, set against another man's shabby one, and the glittering cargo a doomed city can no longer buy. Greek did not feel a jump there; English readers do, because our translations have to choose.
Revelation puts that double edge to work. The city that trades in gleaming goods loses them, while the bride is handed linen that is bright and clean, and the last chapter still has a river and a star that shine. The book takes the word the world uses for what money buys and gives it back to what God gives.