Aquamarine Shale is a block added by Astral Sorcery. It is found underwater in Sand that is exposed to the sky. Breaking the block causes Aquamarine to drop. Drop quantity is affected by the Fortune enchantment.
Astral Tome entry
Aquamarine Shale: Shaped by the continuous flow of water and starlight over fragmented rock crystal, these small and fragile gems form in the sands of riverbeds and beaches. Usually they can be found close to the surface buried in patches of sand.