Solid, filled, vermeil, plated

Gold itself doesn't tarnish. When a piece darkens, it's the other metals: the alloy in lower karats, or the base metal showing through a worn layer.

Why it matters

A thicker layer lasts longer against daily wear. Solid gold can be polished and resized; a plated piece can be replated but not polished back.

Definitions from the US Federal Trade Commission jewelry guides. Each piece's own details say which it is.