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My favorite alloy is copper-gold alloy, here is why:
Copper is dissolved easily by acid where gold i not. Therefore if you rub any such alloy with acid you can eat away the copper leaving just the gold. This creates a similar effect as gold plating where a thin layer of pure gold is on the outside and the whole object looks like pure gold.
Of course gold is soft so even though the whole alloyed object is pretty strong thanks to the copper the thin outer layer is soft and wears away over time easily. When it goes from gold back to a rose-gold copper color just rub it down with a little more acid and it turns gold again.
Its much cheaper than pure gold but looks just as nice and is easily cleaned up. Its really win win in many ways.
Interesting note, the romans used to use this for their armor. Since they would clean it off with acetic acid from vinegar it looks like pure gold and thus was impressive on the battlefield to see. The gol shine would wear off fast from battle but was easily restored in the next cleaning.
