The concept of a Reality Marble (固有結界, koyuu kekkai, lit. innate bounded field) is having one's inner world materialized and projected onto reality following the World Egg (世界卵, Sekai tamago) theory. Generally, it is used to refer to the alien common sense of demons (悪魔の持つ異界常識, akuma no motsu ikai joushiki) in that it is how demons perceive reaity and how Reality Marbles themselves are the natural abilities of demons and elementals. It is possible for humans to achieve this as an innate Bounded Field, though generally only in the cases of powerful beings. This ability is a sub-category of Marble Phantasm, but rather than reshaping the world at their will, Reality Marbles allows only for the creation of a set world that defines the practitioner. It can be said that they can surpass Marble Phantasms, as they are capable of affecting things not of nature.
The created worlds are completely cut off from normal reality, projecting their internal landscape and sealing off the regular World. Following the World Egg theory, the actualization of a Reality Marble is the result of switching the Self and the World while keeping the boundary the same. The size of the Self and the World is switched, enclosing the World in a small container while the Self is expanded. The contained World is the World Egg and the name of the theory. They will encase anyone within range, causing them to disappear from view. Sensors are still able to feel the presence of the Bounded Field, and while they cannot be directly affected from the outside, a large burst of chakra may be able to disrupt them. The World constantly attempts to crush the unnatural bounded fields, as they are not created by spiritual beings, meaning that it costs a great deal of chakra to construct one and keep it stabilized. This generally only allows them to be used for a few minutes, but given their powerful nature, that is likely enough time to properly utilize them. Each world has its own set of patterns and rules, which can be said to be an invasion of reality by the diagram inscribed on the soul, and they cannot be freely manipulated by the caster. It is possible for practitioners to make use of certain aspects of their Reality Marbles without activating them. Unlimited Blade Works is the responsible for the Projection and Reinforcement abilities of its practitioners.