White dwarf (WD) atmospheres polluted with rock-forming elements like Fe, Si, Mg, and O result from the accretion of extrasolar rocks. These metal-bearing WD convective layers provide opportunities for characterizing the chemistry of rocky bodies beyond the solar system. I will describe our recent results suggesting that extrasolar rocks resemble those of the solar system both in overall composition and in their high levels of oxidation that is 100,000 times that of a solar gas. The latter is a characteristic feature of rocks in the solar system and profoundly affects the nature of rocky planets. Aqueous alteration of planetesimals is a likely mechanism for the oxidation; in effect, the polluted WDs are evidence for the importance of water in the evolution of terrestrial planet precursors throughout the Galaxy.