how do you get treasure for stockpiles?

[dewey]

is it really only through diamonds? that seems like such a strange and bad decision considering treasure is a depleting resource which you also need for upgrades. theres no map squares that get you treasure either. i have over 200 hours sunk into this game and have not the slightest clue on how to get treasure 

[Catsquid]

There are a few items found in the second and third mines that also count as treasure, but if I remember correctly those are mostly rare gemstones, too.

There actually are map squares that get you treasure, though! But they're at the edges of the map, so they can be hard to get. I've found that if you dedicate a few days to gathering treasure, you can eventually "outrun" the constant drain and have enough to buy some upgrades. You'll probably need to unlock some extra squads first, though.