Cracks

The "Cracks" (隙間) is a name given to a category which certain pieces of folklore, and Magic derived from them, with no clear religious basis or origin fall into.

Principles
For most examples of folklore, the stories that comprise them usually have an origin or a basis which can be traced to a certain religion, mythology or culture. Similar stories might spread through various different cultures, but where they originally came from can usually be found.

However there are certain stories and pieces of folklore which don't have a clear religious basis and have an origin which cannot be traced, despite having spread throughout many different cultures - these stories fall somewhere between the cultures and religions they have spread across, in the "cracks" between them.

The advantage of this unclear origin and vague rules from a magical perspective is that although spells based on such folklore might not as easy to control as ones based on folklore with a clear origin and rules, in having vague rules they have the possibility of breaking through the upper limit that the spells with clear rules would have.

Toaru Majutsu no Index SS: Necessarius Special Admission Test
Two Magicians who use folklore that has fallen into the "cracks" as the basis for their Magic, appear during the events surrounding the Amakusa's admission test for Necessarius; Freadia Strikers, who uses Magic based on a cursed jewel which kills its owners, and Emilie Fordia, whose Magic is based on the pair of a sword and a healing stone.

Examples of folklore that has fallen into the "cracks"

 * A cursed jewel/precious metal which kills its owners: This story concerns a cursed jewel or precious metal which, despite having no real history to it, would kill each of its owners.
 * Freadia Strikers makes use of this curse in her crystal radio, which has had its crystal replaced with a version of the cursed jewel/metal and transmits its curse to either kill or incapacitate.
 * A sword paired with a healing stone: There are many stories concerning weapons which are strengthened by special stones. One variant of these has a pair consisting of a sword and a healing stone - the injuries dealt by the sword cannot be healed unless the healing stone is applied to the wound. This stone is sometimes part of the hilt, sometimes attached but removable, and sometimes in a small bag tied to the hilt. This tale has spread through cultures across Europe, including Celtic and Norse cultures.
 * This piece of folklore is used by Emilie Fordia who uses a spiritual item consisting of a sword and a scabbard with a set of dials on it which have special stones in them. The arrangement of the stones in the dials produces a specific healing stone, drawing out the properties of the sword corresponding to it.

Trivia

 * The "cursed jewel" seems to be a reference to various famous jewels, such as the, the , the , the , and the , all of which have been surrounded by unsubstantiated legends of reputed curses that brings misfortune and tragedy to people who own or wear them, often leading to their deaths. There are strong indications that such stories were fabricated to enhance the stones' mystery and appeal.
 * The "sword paired with a healing stone" seems to refer to swords like and Whitting, two swords from the  of Norse mythology, with any wound made by those swords said to not heal unless rubbed with the stone they are paired with.