Posted by leah on March 8, 2004

Q: hey there, just wondering what makes certain minerals glow specific colors under blacklight. (i.e. why does amber glow orange and not blue, & why do some minerals like fluorite glow either white, yellow, or blue depending on the stone?)

and why do some animal eyes reflect yellow, for example, while others reflect green?

i've been having trouble finding information on these things, so any information you can find on these topics would be greatly appreciated! thanks!

A: I really can't answer your question on minerals as that's out of my area of expertise. I can, however, give you some info on the eye reflection.

Eyeshine, the reflection of colored light from an animal's eye, is due to a reflective layer in the back of the eye called the tapetum lucidum (ta-PEE-tum LOO-sih-dum). This layer, which literally means "bright carpet", is found in most nocturnal animals or animals in living in dim environments (like underwater). In many of the hoofed-mammals, the tapetum lucidum is composed of glistening connective tissue fibers while in many carnivores and sharks, it is composed of fiber-like crystals of guanine. As light passes through the eye, various layers in the eye filter the light, causing different colors to be reflected as eyeshine in different kinds of animals. The eyeshine of a bear may be a deep red, a deer's glows with an orange-yellow light, while a cat's may show a greenish tint.

John Wiessinger ( March 9, 2004)