The Fire Crystal is one of the four crystals hidden throughout the environment as of Alpha 0.0.82. It is labeled as Fire Crystal (Unknown), possibly because a lighting effect similar to the Light Crystal or Dark Crystal is not yet implemented.
Location
The Fire Crystal is found inside the Blacksmith's shop. It is hovering in the flames of the torch to the left of the main shop door. This makes it the easiest crystal to find and often times most player's first crystal.
The Fire Crystal as it floats in the Blacksmith's Shop as of Alpha 0.0.82.
Touching the Crystal
Unlike the two named crystals, the Light Crystal (Protection) and the Dark Crystal (Fear), the fire crystal has no unique visual effect upon picking it up. It also remains labeled as Fire Crystal (Unknown). This is possibly because it's development is not complete. This is similar to the Ice Crystal.
The Fire Crystal and it's label, being held in the Blacksmith's shop.