From DwarfFortressWiki
Rose Gold
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Rose gold is an alloy of gold and copper, made at a smelter. It is made by combining three gold bars with one copper bar, which results in four rose gold bars. The value of this is a little subjective, as the total material worth of the produced metal is the same as that of the ingredients (3*30 and 1*2 -> 4*23). Disregarding fuel, you lose the possible utility of the copper bar to get purple-coloured building material. If you have no magma access then the transition will cost you fuel as well, so it's really a question of how important it is that you get purple metal.