The iron from meteorites has a very different chemical composition compared to iron extracted from terrestrial minerals. More specifically, the iron meteorites

The iron from meteorites has a very different chemical composition compared to iron extracted from terrestrial minerals. More specifically, the iron meteorites are made of an alloy composed of iron, nickel (5.7% on average), cobalt (0.28% on average) and germanium (0.017% on average), with much smaller quantities of other elements, like gallium, chromium, platinum-group elements and gold. Besides these, there is a certain group of meteorites whose iron alloy also contains up to a few percent of silicon (those come from bodies with too little oxygen, where not all the silicon was oxidized to form silicates). The reason for why the meteorites have this composition, which must resemble the core of the Earth, is that these are the elements that remain uncombined with oxygen when there is not enough oxygen to oxidize everything, so only the elements with a greater affinity with oxygen are oxidized and form a mixture of silicate rocks, while the remainder dissolves in iron. So this very unusual chemical composition is very easy to distinguish from terrestrial iron. Mixing iron with nickel is not enough to simulate it, because there are also the other elements present in smaller quantities, like cobalt and germanium, which show whether an iron alloy is terrestrial or meteoritic. After Earth had formed, it was melted by many impacts and then the heavy iron alloy went to the core, while the lighter rocks floated above it forming the mantle, like the slag in a furnace, and from the mantle, the lightest rocks, i.e. the granitic rocks, raised above the mantle, forming the crust and the continents. Most of the iron remained in the core, and that must have a composition similar with the iron meteorites, while a small part of the iron was oxidized, so it mixed with the rocks. This is the iron that we can find at the surface of the Earth, and when it was oxidized it was not accompanied by the other chemical elements dissolved in the Earth's core, because those are more resistant to rusting.