Unlike most diamonds, which form about 150 kilometers below Earth’s surface, these unusual stones originate at depths greater than 300 kilometers. They preserve tiny samples from deep inside Earth that would otherwise never reach the surface. One inclusion stood out because it appeared to contain iron hydroxide, something rarely expected inside Earth’s mantle. These are conditions found between about 900 and 1,250 kilometers beneath Earth’s surface in the lower mantle. As the mineral gradually changes into other iron-rich minerals, it releases the water stored within its structure deep inside the mantle.