Gold is the reason the seven-metal scheme has a top, and the reason is physical before it is symbolic. Gold occurs native, as visible metal in rock and in river gravel, so it was collected and worked long before anyone understood smelting, and it is almost chemically inert, which is why buried gold comes out of the ground looking as it went in. Every claim the tradition makes about incorruptibility rests on those two facts rather than on anything added to them.
In Egyptian religious language the flesh of the gods is gold, which is why gilding was applied to coffins and to divine images as a statement rather than as ornament. In the Vedic material hiranya carries the sense of light and of deathlessness, and gold is placed with the dead. Christian practice reached the same place by its own route, through the gold ground of the icon and the gilded reliquary, where the metal declares that what it surrounds does not decay.
Alchemy put gold at the end of the work, and what the alchemists meant by that has been argued since the seventeenth century. Chrysopoeia, the making of gold, is described in the technical literature in operational terms, with furnaces, weights and yields, and a great deal of it reads as laboratory practice. Later readers, and a good many later practitioners, took the whole sequence instead as an account of the operator's own transformation. The atlas records both readings, because the surviving texts support both and their authors were frequently ambiguous on purpose.
Sources The Secrets of AlchemyLawrence M. Principe with The Complete Gods and Goddesses of Ancient EgyptRichard H. Wilkinson