A Soul Gem is created when a Wishborn is granted their wish from Kyuubey; in the barest sense, the person's soul is removed from their physical body and placed into the created Soul Gem, with the intent of their physical body then being able to withstand more punishment due to the lack of the fragile soul being housed within it. This piece of jewelry is given a hue that represents the Wishborn best, and is typically their preferred color, but this is not always the case.
The Soul Gem acts as the heart of a Wishborn, both literal and metaphorical, as it must remain on or near the Wishborn for them to remain conscious; removal or theft of the Soul Gem beyond a certain distance will render the Wishborn comatose, and to anyone nearby, it would seem as though death had taken them. Returning the undamaged Soul Gem will alleviate this effect. The gem is not sturdy on its own, and can break just as easily as any piece of jewelry, so it is best kept out of sight when not in use. When a Wishborn is "transformed" (meaning their powers are unleashed and ready to be used), the Soul Gem is often placed on their body somewhere as a physical adherent. If a Soul Gem is destroyed, the Wishborn is immediately slain, akin to having their soul destroyed. Reploids seem to have a different effect when their Soul Gem is destroyed, reverting to a soulless being with minimal automotive functions and an innate desire to seek revenge for what was lost, retrieve that which was lost, or a combination of both. This process is not yet fully understood.
Through use of their power, subjection of bodily harm while transformed, and the stress of situations, a Wishborn's Soul gem will become corrupted, the rate of which depends on how much they focus their powers and how deeply they are subjected to injury, stress, and grief. Grief Seeds, obtained from defeated Witches and Yoma, are required to cleanse the corruption that plagues Wishborn. If a Wishborn does not cleanse themself by the time a Soul Gem is completely corrupted, they will succumb to despair and grief themselves and become a Witch or a Yoma, gender depending. As the corruption builds, Wishborn may begin showing a change in nature to reflect their corrupted souls, ranging from consistent pessimism to lashing out at allies for the smallest of issues. It is advised not to let corruption stretch too far, as it builds more quickly once a Wishborn's emotions begin to fall prey to negativity more frequently and more often.
A Wishborn's Soul Gem also acts as a beacon and warning signal when Witches or Yoma are nearby, flashing and pulsing with increasing intensity as the corrupted individuals grow closer. They will attempt to direct the Wishborn toward the source of either the Yoma or the entrance to the Witch's labyrinth, but have limited range; those Witches or Yoma who are a considerable distance away will not be picked up by the Soul Gem's warning signal.