Probably more important, it's chemically inert (so it doesn't rust away to nothing) and it's easy to assay, at least once you can make aqua regia, an ability which was mastered by the 13th century CE, and possibly by the 14th century BCE. Harappan touchstones have been recovered, and although cupellation can't separate gold from silver, it can certainly separate it from iron, lead, tin, and copper, and we've found cupels with litharge deposits from the 4th millennium BCE.

Cupellation plus touchstones are adequate both to refine electrum and to destructively measure the quantity of gold in the electrum. By Roman times, we were smelting electrum with salt to separate out the gold by chloriding the silver, and the Lydians may have been doing that already by the fifth century BCE, when they started debasing their electrum coins with extra silver.

This sounds kind of stupid, since at this point we're used to being able to purify and quantify all kinds of materials, but in fact these attributes made gold and silver unique among rare elements for centuries.