According to the wiki, they come from Nimbus Sealed Chests. However, me and my friend have opened enough to get the healing scrolls for a full regen build, plus some extra, and we haven’t gotten a single aventurine.
Aventurine’ Banner is on rn, but you need 180 pulls for a garantee so better get grinding for those stellar jades (im pretty sure you can get it from charts)
so to get adventurines there’s this OP unknown trick where you join a person named “camreeyan” and if you send him every single enchantment scroll you own he will trade you 999 adventurines