Nor does glass or plastic-coated aluminium. There are lots of shiny things that people don't want in their jewelry. If diamond were as common and cheap as glass, we'd probably see just as much diamond jewelry as glass. They're not massively "better" at being shiney. Even artificial gems are worth less in than natural ones even if they have fewer defects!
Neither of those things were around or refined enough like gold was when gold became the traditional metal for jewelry. Gold has a large first-mover advantage here that is being propped up by human psychology.
