Most sages who know anything about these minor artifacts agree that they were first made by dragons who learned the art of crafting enchanted tools. Other cults over the eons have created one or two for high priests and hermit-like oracles, but dragons and those wizards who would win a dragons favor mastered the art generations ago. They work a little bit like pearls of power or a momento magica, but instead of recalling cast spells, they hold spells that allow their owners greater flexibility in their spell selection. They can be focused to hold either divine spells or arcane spells, but no jade skull can hold both. Also, their function depends on whether the user is a spontaneous caster, or one who must prepare spells.
In a way, the skulls could be seen as spell batteries. They hold spells that are cast into them, and allow their owner access to them in different ways. A command word opens the skull to accept new spells cast into it. a second command word seals the skull again. There are three different power levels of jade skulls, : least, lesser, and greater.
A least jade skull can store up to 3 spell levels. This can be in whatever formula suits the owner, be it 3 first level spells or a single third level spell, or even a second and a first level spell. For this purpose, 0-level spells take up one spell level each. Once opened, any caster can cast a spell of the appropriate type ( either arcane or divine) not necessarily the owner. Once the skull is prepared, the owner can draw from the spell power contained within. A spontaneous caster may treat any spells stored in the skull as if they were spells known, and draw from them when casting their daily allotment of spells. A caster who prepares spells may give up a prepared spell of the appropriate level to instead cast a spell stored in the skull. In both cases, the spell slot powers the magic, and the spell remains in the skull. A caster must be high enough level to cast any spell drawn from a skull, and the spell must be on the casters class spell list.
A lesser Jade skull works exactly like the least version, with slight differences. It can store up to six spell levels, but it can still hold a maximum of only three spells.
The greater version may store up to nine spell levels, but again is limited to a maximum of three spells.