The strength of lighting is that it's a force multiplier for the brain. You're no longer limited to daylight, which means that when the sun sets and the fields aren't calling you over, you can read. This lets you turn your down time into education, which is *massive*.
(Modern example using solar lamps
HereLight also brings security when used around the edge of communities, letting you see the things that go bump in the night, reducing the number of nocturnal predators that your people fall prey to. IIRC, Earthdawn had most people moving underground, or at least into underground shelters to hide from the Horrors; light around teh edges, in this case, would be bad as the outsiders could see it, but deep inside would be fine. There're likely to be "airlock" designs, with twisting entryways, that allow guards to be stationed relatively near the front with light, but where the light wouldn't shine out into general view. A simple zigzag would be enough.
So, some would be found at the entry of cave systems, then vanish, then be found deeper inside, in the private sleeping areas of the people (Who would also have had simple covers, like baskets or wooden 'domes' that were long-since rotted away that would have covered the light sources in times of olde.) ... there's likely to be one building, either a place of worship or a central library, which would have had a LOT more of the lights, so it coudl be used by large numbers of people at all hours of the night.
In theory, the lights would all have winked out as the mana levels dropped, but you can have them go 'dormant' instead, or have one area where the mana levels never dipped all the way and it was juuust enough to keep some glowing crystals 'alive' through the downcycle. In this way, you can have assorted crystal rocks having been found all over the place for forever, but no one knew what the significance was and assumed people gathered them because "They were pretty", until someone in the modern world discovers the one place where they *work* and goes "ah HAH! Hey! Check it! We finally uncovered what these damn things were all along!"
