tl;dr: "Diamonds can trap small bubbles of extremely dense pressurized water when they form. Then, as the diamond moves up through the mantle, the water inclusi

tl;dr: "Diamonds can trap small bubbles of extremely dense pressurized water when they form. Then, as the diamond moves up through the mantle, the water inclusion is subjected to cooler temperatures while remaining under the same pressurized conditions. In that very specific case, ice-VII can occur."