The Underdark is home to rare gemstones called Luminescent Crystals that naturally radiate light. Each one has the brightness of a candle, but their illumination magnifies when they are brought together in clusters.
The most common of these crystals are clear, and glow with a soft white light. Some crystals are tinted in shades of pastel blues, greens, pinks, and other hues, and glow with a corresponding light. The light is continuous, even when the crystals are removed from the Underdark. When used to illuminate homes, they are typically placed in non-sleeping areas or in sconces or lanterns that can be hooded when darkness is desired.
These crystals are rarely found in the Upperdark, perhaps being mined out already. Dwarven and gnomish miners are more likely to find them in the Middledark, where humans rarely delve.
Dwarves and gnomes often trade these crystals with wealthy surface-dwellers who use them in chandeliers and wall sconces to magically light their castles at night. They also use them in objects of art — as the glowing eyes of a dragon figurine shaped from gold, for example, or different shaded shards used to form a glowing mosaic.