Grinna Amber

You know Grinna Amber? It is these rocks they are gathering like crazy up in Padaccia to make some weird lamps out of it. How stupid of them. That amber is basically worthless what little light it gives off, can't even do proper magic with it. I see no practical applications for this small glowing rock whatsoever.

Reported conversation between dwarven Magnat Zuntan Cobaltaxe and his associate
A year before he almost ruins himself trying to buy the most profitable Grinna Amber quarries

Grinna Amber is a slightly magically glowing stone that can be found in the area of the Coppania River, especially in Coppania valley and the Cosania Lake. Furthermore there are some deposits in the mountains of Apac's Sword, usually in the vicinity of underground lakes, as the dwarves report it.

The Amber is by nature able to infuse itself with magical energy it consumes from light shining upon it - comparable to photosynthesis of unmagical plants. When the magic reaches a certain limit, it starts to glow on its own in a slight yellow-orange color. This glow effect can last for hours and has a constant intesity over time.

Although it apparently can contain a rather large amount of magic, it is almost unusable as a magical storage or catalyst, since its magical transmission properties are simply too low. For unknown reasons it is not possible to transform the magical energy into heat instead of light.

However, it was discovered that in combination with a certain gas, the rate of light given off by the Amber can be increased significantly, once a short burst of magic was given into it. Naturally, the amount of lowing time is then reduced to a few hours, but a full day of light usually is enough for a recharge. Based on this concept, the Daylight Crystallamps utilize Grinna Amber as lighting material.

Many animals in the vicinity of Lake Cosania have ingested small quantities of Grinna Amber and as a result seem to glow themselves a bit. The most prominent example is the Glowcidae that keeps the amber in its boy, enabling it to glow during the night.

The negative transformation of the Old Coppania Lowlands after the Coppanias was diverted in Padaccia seem to suggest that long term exposure or the presence of certain other materials can have negative or even harmful effects.