To utilize Fire Crystals in Clash of Clans, you must first reach Furnace Level 30 and be on a server that has entered the first Fire Crystal Age (occurs 60 days after server launch). Once these requirements are met, Fire Crystals can be used to upgrade buildings through the Fire Crystal Ages. The number of Fire Crystals required for upgrades increases with each age.
To obtain Fire Crystals, you can utilize the Crystal Laboratory. Upon reaching Furnace Level 30 and entering the first Fire Crystal Age, you can build the Crystal Laboratory. It allows you to earn 5 Fire Crystals daily by refining 25,000 each of Meat, Iron, Coal, and Wood. This is an efficient method for acquiring Fire Crystals without spending money.
Additional ways to obtain Fire Crystals include participating in events, purchasing them from the shop, or using the Builder Base Forge to exchange resources.