Imperfections can reduce a diamond's value to a jeweler, but they may render it priceless to a geologist. Take the tiny speck in the diamond above; too small to be visible to the naked eye, it could help settle a long-standing debate about the amount of water in Earth's mantle. Down to about 400 km below the surface, the mantle is mainly a mineral called olivine, which does not absorb water. However, below this, the immense heat and pressure cause the olivine to adopt different chemical structures, one of which is called ringwoodite, which laboratory tests have shown can contain up to 2.5% water. The chemical structure of the diamond above, unearthed by magma pushing its way to the surface in the Juina district of Brazil, shows that it was formed more than 400 km deep. Under a microscope, the researchers spotted a 40 micrometer crystal trapped inside the diamond called an inclusion. Spectroscopic analysis showed this to be ringwoodite. Further analysis detailed online today in Nature shows the ringwoodite contains hydrogen-oxygen bonds, which suggests the crystal lattice contains at least 1.4% water. The place where the diamond was produced may not be typical of the entire lower mantle, but if it is then there could be a lot of water down there. This would be important, as changes in the temperature in the mantle could cause it to expel highly pressurized steam, which could lead to volcanic eruptions.