Crystal cores are mostly used to make Condensed Resin, an important part in the game’s resin economy.
Picture: Crystal Core.
Condensed resin recipe allows you spend your Original resin to farm gold, experience and some materials twice as fast. It also allows you to save resin for the day when there is an opportunity to farm what you want.
As a result, people make a lot of condensed resin (you can make about 4 per day), and need the crystal cores to make it happen.
Crystal Cores are also used to craft a couple of potions and Adepti Seeker’s Stove (a portable cooking station), but it’s rare to actually need these things.
Crystal Cores are obtained by catching Crystalflies, which many players find frustrating. Personally, I find them easy enough to catch if you aren’t aiming for perfection – I catch about half the Crystalflies I attempt to catch, and there are places with plenty of them.