The Celestial Crystal is an item added by Astral Sorcery. It is used as a superior counterpart to Rock Crystals, and can reach a size of 900, which is 500 more than Rock Crystals.
The Celestial Crystal can be acquired by mining a Celestial Crystal Cluster after it has fully developed. Trying to grow a max size Celestial Crystal in Liquid Starlight has a 1 in 6 chance to split a crystal into two, of which one will have 0-9% additional purity.
Astral Tome entry
After having the stardust completely fuse and tint these blue-ish crystals, they still seem to share the same properties as rock crystals, but are bigger and more robust than their white counterparts.
Studying them further, they seem to resemble the rock crystals in many parts, but exceed them in almost every aspect. Using them instead of the smaller ones might be very advantageous.