Mercury
Diamonds are forged under high pressure and temperature. On Earth, they are crystallized deep underground then ride to the surface during volcanic eruptions. Studies of asteroid/comet impact suggest that their impact also creates high pressure and temperature that can transform the carbon into diamonds.
Mercury has a history of large asteroid/comet impacts and its crust is mostly made up of graphite. A pure form of carbon that can convert into diamonds. As per the simulations based upon impacts on this planet. Mercury could have up-to 16 quadrillion tons of diamonds, over 16 times Earth’s estimated reserves.
Credit Mr Scientific