The appearance of the white layers, having become spotty and grayish-looking, is due to glass decay. This is not so unusual among 16th C. chevron Beads.

I would not say this makes your bead "more rare" than a green 7-layer bead. But, in all likelihood, 7-layer green beads are not as rare as some people would like to believe.

The idea that the exterior color of your bead is or was "teal" is mistaken, of course.

Jamey