Skill crystal

Monster Cards/Crystals is a rare drop from demons.

Appearance
Monster Cards are about the same size as Intelligence Cards. They also appear to be blank. In the Manga; these are Monster Crystals instead of Monster Cards.

Description
Monster Cards/Crystals are demon drops that, when fused to a piece of equipment, will bestow upon that equipment a skill that depends on what kind of Monster Card/Crystal is used. Only a Master Smith can perform Monster Card/Crystal Fusion. Monster Cards/Crystals can only be identified using a Guild Temple. They first appeared in Web Novel Chapter 45.

Trivia

 * Monster Cards/Crystals are very rare drops, as it seems that there aren't a lot of them in circulation; the only other way to get them is through auctions.
 * There are Monster Cards/Crystals that are called high-ranked Monster Cards/Crystals that can only be used when fused to an equipment together with a High Kobold Monster Card/Crystal.
 * Most people believe that Monster Card/Crystal Fusion relies on luck. This is false as a Monster Card/Crystal Fusion will always succeed as long as the item has an empty skill slot. If there are no more empty skill slots, then the fusion will fail, the Monster Card/Crystal will disappear, and the item will be reduced to its raw materials.
 * Michio uses Appraisal to tell if an item has an empty skill slot or not.