First of all, the diamond ocean is not only found on Neptune, but also on Uranus.
Under the high temperatures of Neptune and Uranus and pressures 11 million times greater than earth’s, the diamond ocean changes from solid to liquid, covering the planet’s surface and forming an ocean. This strange ocean contains huge, solid “diamond mountains” the size of icebergs. But for now, the diamond ocean is just scientists’ guess as to why the magnetic fields of Neptune and Uranus are bending, not confirmed.
So it’s true that there is a diamond ocean on Neptune.