Aquamarine is a gem/ore that is used for various crafting recipes in Astral Sorcery. It can be found relatively commonly along just about any land biome that connects to water in some way, as they spawn in the sand, They can also be used to get Liquid Starlight in a Lightwell, but this isn't advisable since they tend not to contain much and drain slowly.
Depending on mod configuration, Aquamarine Shale will rarely be generated instead of Sand when Lava touches Liquid Starlight.