Whyyyyyy the glowing crystals always, whyyy-yy-yy why whyyyyyy??!?!" /me the geologist breaks down sobbing and shaking fists at the sky then realizes she is on a public forum and pretends to be stretching and yawning...
But seriously, not only are glowing crystals as a sole lightsource geologically unrealistic, they're also kinda overdone...Everyone seems to want to have glowing crystals in their Age. But look at the great lighting in Riven--it was all very dramatic and moody, but not a single glowing crystal around.
*Disclaimer: Then again, maybe there are giant glowing crystals in Riven, I wouldn't know as I still haven't finished it yet...