TERA's new Crystal Fusion game mechanic is one way of getting the newer tier of weapon and armor crystals (the other methods would be through direct purchase through vanguard reputation vendors, and via crystal boxes offered as weekly bonus rewards for Vanguard quests).
You can combine crystals (i.e. cruxes) together using a button at the bottom of your inventory to create a new crystal of the same tier, or a chance at creating a new crystal of a tier higher (niveots) using a Niveot Structure (a blank niveot crystal, which can be purchased from the Vanguard Initiative Quartermaster). The process requires three armor, or three weapon crystals of any kind and you can mix and match types. The resulting crystal is determined through random number generation (RNG).
If you have a lot of spare crystals lying around, you can get your first Niveot Structure by completing the Fusion Bomb achievement, which rewards the player with one Niveot Structure. This achievement required you to perform 50 crystal fusions
When using a Niveot Structure there is a chance that the fusion will result in failure. When this happens the Niveot Structure is consumed.
This process can also be used with jewelry zyrks to create vrysks.
The rare (blue) unbreakable version of the crystals can only be created by fusing three uncommon (green) niveots with a Dyad Niveot Structure. This also works off of the aforementioned mechanics and uses RNG. The new crystal is not able to be traded, but can be stored in your character bank. Creating one of these crystals also adds a secondary effect to the crystal.