Soul gems are items that allow the player to capture monsters, these captured monsters can act as guardians/attackers or used for Transformations.
They have 6 levels, each being able to capture different monsters, every higher level gem can also capture lower level monster.
Souls gems from level 1 to 5 are both craftable and found in chests around the world, while gems level 0 are created by extracting chickens/cows from mangers and paying 1000 souls and gems from level 6-9 are not implemented yet.
At higher levels, monsters become increasingly difficult to catch, requiring many tries depending on how low their health is (the lesser health a monster has, the easier it is to capture). This can be fixed by using higher level gems, which make capturing monsters more likely.