In the case of gold alloys, you might wonder where the silver atoms go. The answer is that they sit inside the gold crystal structure, taking the place of a gold atom, and it is this atom substitution inside the crystal lattice of the gold that makes it stronger. Alloys tend to be stronger than pure metals for one very simple reason: the alloy atoms have a different size and chemistry from the host metal’s atoms, so when they sit inside the host crystal they cause all sorts of mechanical and electrical disturbances that add up to one crucial thing: they make it more difficult for dislocations
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Because Gold itself is a soft metal, most jewelry isn't pure gold but instead has a small percentage of silver or copper. Not only does adding an alternate metal make the final product harder but it also changes the overall color, with silver making the gold whiter and copper making it redder. The more they're used, the more pronounced the color becomes.
