Celestial crystals are enhanced versions of the regular rock crystals that can be mined deep underground. They are created by throwing stardust and a rock crystal into a pool of liquid starlight where a Celestial Crystal cluster will form. After a while the Celestial Crystal cluster will become fully grown and can be harvested by simply mining it with a pickaxe. Don't be hasty though because breaking a cluster before it is ready will either give you a low quality Celestial Crystal or nothing at all!
The growth process of the celestial crystal can be accelerated by creating it on top of a starmetal ore. It will absorb the starlight infusion that transformed the iron ore into starmetal in the first place, thus returning the starmetal back to its original iron ore form. Making the space the iron ore occupies the target of starlight transmutation, however, will continually turn it back into starmetal ore so that the celestial crystal cluster can continue growing.