The universe is very far from being chemically uniform, which is a result of the way the universe developed. The earliest stars, perhaps thirteen billion years ago, condensed from a thin gas of hydrogen. As a star condensed, the heat generated in its center triggered the hydrogen fusion process, in which four hydrogen nuclei coalesce to a helium nucleus with the release of a large amount of energy. The star "switched on." In big stars with sufficient ability to retain heat, higher temperatures in the center led to the successive triggering of helium fusion and then to reactions producing the heavier elements. The stars were the "crucibles" in which the heavier elements were formed. Supernova explosions scattered these elements through the cosmos as debris from which, in time, the second-generation suns such as our own formed.

Our solar system was thus enriched in heavy elements from its very beginning. As the sun coalesced at the center of a cloud of diffused matter, so did the various planets farther out. The large planets with enough gravitational pull to retain all of their original material, such as Jupiter and Saturn, remained sunlike in overall composition. The Earth and the other small inner planets had their volatile elements driven away by the heat of the sun and by the weakness of their own attraction for them. The only substances left were the nonvolatiles; thus Earth became a denuded ball of rock. This is why our planet is so rich in silicon-oxygen minerals today; these were the substances that would not boil away.