(Magicite is relevant to Final Fantasy 6 worlds only. It can probably be used in other worlds, but care must be taken as it replaces the standard spell acquisition system.)
Magicite is the form that can be taken by Espers when their bodies die. This conversion is typically voluntary, though magical techniques have been developed to force an Esper to take magicite form.
The Gestahlian Empire has, for decades, been draining the magical powers of Espers captured during the raid of the Esper World. Great care has been taken to maintain this draw of power as long as possible, and to not kill the Esper in the process. While this method does, indeed, generate a good deal of magical energy, magicite in its purest form is far more efficient.
Magicite is a smooth, green stone with a deep red glow at the center. A stone is a Diminutive item weighing 1 lb.
Magicite is most effective for mages, as it replaces the typical 'scroll' spell acquisition. When attempting to learn a spell from magicite, the mage must be able to learn the spell: the spell must appear on the mage's spell list, and the mage must have the appropriate level to use the spell. If these requirements are met, the mage must then attempt to commune with the Esper inside the magicite. The mage makes a Spellcraft check (DC 15 + twice the spell's level). If this skill check fails, the character cannot attempt to learn that particular spell again until the next day. Once the mage has connected with an Esper for a particular spell, they do not need to make the check for that spell again. If a magicite has multiple spells, the mage must roll separately for each spell.
Once connected, a mage may attempt to learn the spell from the magicite. Once per day that the mage is attempting to learn a spell, they make a Spellcraft check (DC 15 + twice the spell's level). The mage needs a number of successful checks equal to the spell's level to successfully learn a spell. For example, a mage trying to learn a level 3 spell must make 3 successful Spellcraft checks to learn the spell. These checks do not have to be consecutive, but the mage may only roll for an individual spell once per day. If a mage is attempting to learn multiple spells, the Spellcraft check DC increases by +1 for every spell beyond the first on all checks.
Classes who do not normally cast spells may use magicite to gain a modicum of spellcasting ability. Any non-caster class who gains access to magicite immediately gains an MP pool. The MP dice are rolled as follows:
1-4: 1d3-1 5-8: 1d4-1 9+: 1d6-1
This pool is rolled retroactively, so a level 4 player first witnessing magicite will roll four times at 1d3-1 to account for not having an MP pool. It is possible for a character to gain no MP as a result of rolling.
Non-casters gain no bonus MP from a high attribute, and all spells they cast always function at the lowest possible level. For example, a non-caster who learns a spell normally available to a Black Mage at level 3 will always cast that spell as if their caster level was 3. As well, MP costs for all spells are double their highest possible cost. These characters may commune with Espers in the same way as mages; however, if they do not have Spellcraft, they may use the Use Materia skill at a -4 penalty, or the Concentration skill at -8.
These rules apply to anyone learning a spell that they do not legitimately have access to. A Black Mage, for example, may learn White magic spells, but must follow all of the above rules, and is still limited to spending no more MP on a single spell than half their caster level. If a spell may not be legitimately cast by the character, metamagic feats cannot be applied to the spell.