A magical crystal that can be found in the soil of the Demon Realm and appears in the Monster Girl Encyclopedia World Guide II book.
Guide Book Entry
Found in the soil of the Demon Realm. this is the sought after crystal that can be mined. Able to contain more magic than Demon Realm Silver, these gems aren't only an ornament but also a common material used in the creation of magic items. Naturally a clear and colorless gem stone, when infused with a monster or human's spirit energy, the crystal's internal structure and color will change depending on the magic stored within it. Each owner's gem shape and color being unique to them. Thanks to this, with some study it is possible to discover who crafted an item with these gems.
Once a devil gem has had a spell cast onto it, it's impossible to load a different spell onto it. Since a Devil Gem can't be changed after the first spell cast has been cast on it, a great deal of care must be taken while excavating them. Also, because of this, the cutting of devil gems should be left to the most skilled of craftsmen. The spell cast on the devil gem should also be cast with the utmost care, using the greatest magical power and purity possible to create the best class of gem possible. If a Devil Gem hasn't been cut properly, or a spell is cast onto it badly, it' possible for the gem to shatter.
Because of the preciousness of Devil Gems, the Demon Lord and her husband are known for producing many of the greatest Devil Gems now in existence. No matter if they are human or monster, there are many fervent collectors of their Devil Gems around the world. From the Generals of the Demon Lord's army to the Pope of the Church and even it's saints, those who possess these devil gems are able to display them with pride.
Engagements Rings
The engagement ring is a ring to be exchanged on the day of marriage, typically in a temple of the gods. Originally this wasn't a part of monster culture, but now that human and monster culture is intermingling, the worship of the Goddess of Love has become a common sight on both sides. As a result of this, the tradition of exchanging wedding rings has also found it's way in. When the marriage includes a monster, the rings often incorporate Devil Gems. Two Devil Gem rings, loaded with one another's spiritual energy, are exchanged between the groom and the monster bride. The groom getting a ring infused with his bride's energy and the bride getting a ring infused with her soon-to-be husband's energy. Both the bride and groom are able to study their ring's magical content to gain a deeper understanding of one another.
While not required for a marriage, engagement rings that incorporate devil gems are an item that many monster couples desire. As such, monster girls have been known to go to great lengths to retrieve the perfect gem for their husbands. For many monsters, expressing the love they have for their husband brings them great joy, hence their dedication to finding a fantastic gem. They are also able to sense how much magical energy their husband has while he's wearing his ring. The color and radiance of the Devil Gem varying depending on his health and mood. Also, while rare, it is possible for the Devil Gem to lose it's luster and transform in a plain stone. However, this will never happen to a monster couple in a healthy and loving relationship. As long as the couple continues to have sex, the wife's magical energy will continually recharge both of their rings to keep them glistening and wonderful.