There are 4 crystals
1.Light Crystal-Found near the blacksmith shop where the stone spear and shield is
2.Fire Crystal-Can be legit found near your blacksmith place in the corner torch
3.Water Crystal-Can be found on water ofc
4.Dark Crystal-Found in the small house can let you see the Lord Of Darknes/Lord Of Fear/Warrior Demon etc.
If you put the Light Crystal and hold the Dark Crystal the black thing won't appear since the Light Crystal is the Crystal of Protection idk what the other Crystals do tho maybe the crystal's will be the main plot of the game in the future so yea this is all i know rn lol