What is a soul gem capable of?
A Soul Gem is the physical manifestation of Uunice's Magic, and was used in the creation of the Miraculouses and Miracle Boxes.
A Soul Gem is able to harness and manipulate the magic of nearby kwamis, and change the magic surrounding the Miraculouses with it. Some examples are Trixx's Deception being used to hide holders' identities, Tikki's Creation to summon the weapons and suit, etc.
The last time a Soul Gem was used was with the creation of the Celestial Order of the Guardians, where they used the Elephant Miraculous (Memories) and the Bee Miraculous (Subjection) to change the magic surrounding kwamis. They used Pollen's magic to make it so kwamis are bound to the orders given to them, and Tuskk's to remove their memories of things relating to the Miraculouses (ingredients in potions, how to fix Miraculouses, etc) to give humans more control over the kwamis.