War maps are easy ways to get constant crystals. They eat them 20 at a time, and everytime they eat, they drop a crystal. Tested on 8 gacha's, 4 eating war maps, 4 eating stone. All max level. The ones eating war maps were putting crystals everytime they ate, ones eating stone took a few times each crystal.
Gachas are essentially living lootboxes and resource farms. They will eat anything (including stone, wood, or fiber) and turb it into crystals which can drop a variety of resources including black Pearl's, organic polymer, or obsidian, among other things. They also have a rare chance to drop higher-tier crystals which turn into upgraded armor and tools