The crystals in the game are needed for various things. Upgrading armor, building towers, Making rare weapons and armor. There are several color crystals and each one serves its own purpose.
Blue crystals are used for upgrading some armors, making weapons and upgrading some skills.
Orange crystals are used for upgrading items that have already been upgraded by blue crystals. (Example: Steel armor gets upgraded to chrome with materials and a blue crystal. Chrome armor gets upgraded to gold armor using an orange crystal and other materials)
Purple crystals (rarest) are used to upgrade some weapons and armor (gold armor) one step further.
Green crystals are used to make knight armor (only way to get it) and to upgrade towers.
List of Crystals
Crystals are very rare and are found only in the treasure chest obtained from the guild mines or you can buy it through the auction. According to the cost green crystal are the cheapest and the purple crystals which are the costliest and are very rare.
Green Crystal Blue Crystal Orange Crystal Purple Crystal