Crystals are quite the mystery in Valheim, being dropped from higher level creatures, but without a clear purpose.
Like many items in Valheim, players pick up Crystals and aren't quite sure what they can do with them. In a lot of games, Crystals are used for a variety of things.
They tend to be used in crafting or for simply selling and making some in-game cash. Valheim has Crystals that may or may not do anything at all.
How to get Crystals in Valheim and what to do with them
Valheim players will eventually find themselves wandering through the Mountains biome. Inside of that biome is a humungous rock creature known as the Stone Golem.
The Stone Golem is typically dormant until they are approached. They are shaped like common stones that are found on the ground and blend in quite well with the scenary.
These Stone Golems can cause quite the jump scare if they attack and a Valheim player hasn't already noticed them. They can ambush a player and do some serious damage before the player can even react.
The majority of the time, a Crystal will drop whenever a Stone Golem is defeated. Valheim players will notice this, as the Crystals are quite shiny. They stand out pretty well against the colors of the Mountains biome.
Upon picking up the first Crystal, there is nothing unlocked. Players don't receive a new crafting recipe or anything, unlike many other items found in Valheim. That is a cause for concern.
As of right now, the developers of Valheim have updates galore planned for the game. What those updates contain are anyone's guess at this point, however. Currently, the Crystals in Valheim do nothing.
They can be stored in a base or held on the character. There is bound to be a purpose for the Crystals at some point, but there is zilch to do with them except admire them until that time comes.