Illumination and not

Light is such an essential facet of human life, but it can also be so cumbersome to come by. Sunlight? only in good weather during the day. Fire? it's hot and uses fuel. Fluorescent light bulbs? no idea what you're talking about. Open a window to the plane of infinite light? a definite improvement, but only some people can do that, and you risk a malevolent light spirit. There is but one solution: a light gem.

Light

Light gems can be lit by any precedent magic, namely: light, fire, before, and life. Once active, they can only be stopped by running out of mana or by antecedent magic, namely: dark, ice, undeath, and corruption. A small light gem can light a room for about four hours; in general, larger gems can store more mana, and the shape of the gem determines how quickly it uses the mana. It is common for castles, mansions, crypts, and even restaurants to have light gems permanently installed in the ceiling. Light gems don’t get hot, but most things heat up when enough light is concentrated onto them, so light fixtures are constructed with minimal contact to gems, and with mirrors behind them.

Dark

Dark gems are especially noteworthy as, like light gems, they do not properly open a window to the related plane; instead they simply evoke the relevant effect until they have depleted completely. Dark gems darken an area, no more and no less, until they run out of mana. Certain underground dark healers make use of these gems to ensure total darkness during healing rituals, but besides this there are comparatively few conventional uses for dark gems.

All gems share the property that anyone capable of using magic may use them by directing mana into them. For ignisible gems (light, fire, ice, dark, life, corruption), mana is stored before use, and the gem must be activated.