Examples of Hymn Crystals: the crystals containing the songs Purger, Linker and Re-Nation.
Hymn Crystals (ヒュムネクリスタル, hyumunekurisutaru?, lit. "Hymmne Crystal") are memory card-like devices that contain a program sealed within themselves. Said program always takes the form of a Song (Hymmnos Extract) for the Reyvateils to sing it.
The Storage Qualities of the Hymn Crystals
The Hymn Crystals are Grathnode Crystals that possess no special qualities and that strictly speaking, possess distortions with equal energy amounts for all wave directionalities. If an Install was attempted using them, the distortions would all offset each other, which means they would cause no distortions on the Reyvateil's Static H-Waves (in summary, their GD value is zero).
The crystal maintains all distortion correction energies in equilibrium, so they would discharge no distortions upon the shape of the FFT Spectra that serves as the balance center for the energy, meaning their state is preserved forever. This preserved FFT Spectra that serves as the balance center itself is the [feelings], or [fully deployed Static H-Waves of the Reyvateil] that are recorded within the Hymn Crystal. The recorded mind of the Reyvateil itself then continues existing without destroying the crystal's internal balance.
The Download Ceremony
The reason why a Hymmnos Spell is chanted at the time of a Download is because power is borrowed from the Tower when the perfectly balanced waves are being recorded into the Reyvateil. If the Crystal was Installed normally, it would be [just like placing inside a feelings egg with shell and all], meaning it wouldn't be able to reach the Reyvateil's mind. In fact, the Hymmnos Spell for the Downloads is actually a command that carries out the process of not just accessing the Cosmosphere from the Binary Field, but also drawing out the preserved FFT Spectra and making it join with proper Static H-Waves.
Why are Songs Crafted Instantly?
The Song gushes forth as soon as the Download is complete. This is because all the axes (all wave frequency bands, all time bands) are instantly and completely overwritten by the FFT Spectra recorded in the Hymn Crystal, meaning that the Spectra's state changes to [in the process of crafting a Song]. If we looked at it from the receiving Reyvateil's viewpoint, to her it would seem like the Song was born... as if she had already had known it previously, a little bird born within her memories and feelings.
The Security Concerns Over an Hymn Crystal
Because Hymn Crystals may contain powerful functions and effects, their Downloads require either a Hymn Code or a high enough IPD level to be used depending on the receiving Reyvateil's Server. However, in some cases this is not enough, which is why some crystals have a license control key similar to the ones used for PC programs in the real world. Once the crystal has been used a certain number of times, it will vanish away. Examples of this are the Purger and Despedia crystals, which given their powerful effects can only be Downloaded once.
In case the Song is too important (and too dangerous to be left alone in a Hymn Crystal), then it is Preinstalled via a D-Cellophane, or simply recorded in the SH Server of the user, to prevent it from being used by anyone else besides the Reyvateil designated to be its singer.