Sandpaper + gem = polished gem seems to work for every gem but rubies.
Also, if I might make an unrelated suggestion, building a void altar is absolutely tedious because of the obsidian shards that you have to crush in the mortar for the bricks, and each one takes ages. Perhaps there could be an obsidian breaker spell in geomancy that has a 1/3 chance of dropping a shard instead of the obisidian block, just like flint drops from gravel. Or a charm that costs a reasonable amount of obsidian shards, if gating it behind pure grinding at the mortar is intentional.
Sandpaper + gem = polished gem seems to work for every gem but rubies.
Also, if I might make an unrelated suggestion, building a void altar is absolutely tedious because of the obsidian shards that you have to crush in the mortar for the bricks, and each one takes ages. Perhaps there could be an obsidian breaker spell in geomancy that has a 1/3 chance of dropping a shard instead of the obisidian block, just like flint drops from gravel. Or a charm that costs a reasonable amount of obsidian shards, if gating it behind pure grinding at the mortar is intentional.