From the study of the geochemical differentiation of the earth, it has been indicated that in its early stages of development the planet must have been in a liquid state. The study of meteorites, the distribution of elements in the whole substance while an ore is treated in a blast furnace etc. give evidences to assume that the earth is having a nickel-iron core.
It is presumed that when all the oxygen and sulphur had been consumed in reactions with the active metals, excess iron would separate in the form of droplets. Because of their comparatively high density these drops of iron would sink through the molten or viscous silicates towards the centre of the planet, eventually concentrating there into a central metallic mass, the core of the earth.
Several of the rarer metals which are not chemically very active, gold, platinum, nickel, for example would react to only a moderate extent with oxygen and sulphur. The excess free rare metals would tend to concentrate in the descending iron droplets and would also ultimately find home in the core.
This presumption goes a long way to explain the nickle-iron (nife) composition of the core of the earth. The exact nature of the core is still controversial.