Ground astral rune is a quest item used in the Dream Mentor quest. It is created by using a hammer on an astral rune and then using the resulting astral rune shards in a pestle and mortar.
Ground astral rune is ingredient in the shared dream potion needed in the quest.
It has no other use thereafter, however.
Trivia
- Ground astral rune is one of the two magic-imbued runes that can only be ground by pestle and mortar, aside from Mud rune.