Convenient Light Source
A byproduct of mining and refining arcanium, glowstones are crystals that contain no significant magic energy, but continually radiate illumination. Glowstones come in the same range of colors as arcanium, but with the exception of white, are equally common (in other words, you’ll find as many reds as yellows.) Each glowstone emits light equivalent to one to five candles, but they do not emit heat. Glowstones range from the size of a man’s thumbnail to the size of a large egg.
Because of their size and durability, glowstones are a favored piece of dungeoneering gear. A variety of arrangements exists for housing and carrying glowstones including jewelry and modified lanterns, but it is perfectly safe to just carry a glowstone in your hand and then put it in your pocket when it’s no longer needed.
The biggest drawback to glowstone use is that there’s no way to predict when a stone will stop glowing. This plus the sketchy reputation of many glowstone merchants means that serious users will have multiple back up stones.