The Crystal Core is about to become a hot item in Genshin Impact, as it will be needed to create Condensed Resin after update 1.1 arrives. The Crystal Core is not a new item, and already exists in the game if you wish to get a headstart on farming it.
You can get Crystal Cores by collecting Crystalfly in Mondstadt, and Geo Crystalfly in Liyue. These are small butterfly-like creatures that you will see flying around in the world upon occasion.
They are both easy enough to spot, as the Crystalfly has a deep, blue-green glow, while the Geo Crystalfly has a deep yellow glow. They do have a pretty large spawn area, so you will need to check some pretty big areas of the map to find them. Thankfully, there are locations that you can check regularly to see if any have spawned.
Mondstadt – Crystalfly
The best place to farm Crystalfly in Mondstadt is at the Statue of the Seven in Windrise. Multiple Crystalfly will spawn here, especially at night time. They can fly around above you, so a character like Venti makes it much easier to farm them. Keep in mind, you should not hit them with an Anemo ability, as this causes them to despawn before you can harvest them. You also don’t need to do anything special, and can just grab them in the air, as long as you can reach them.
Liyue – Geo Crystalfly
The best place to farm Geo Crystalfly in Liyue is at the Domain of Guyun, in Guyun Stone Forest in the southeast part of the map. You don’t need to run the domain, the Geo Crystalfly is a regular spawn outside the Domain at nighttime, and can also be found on the rest of the islands.