Can You Identify This Stone?

(via The Canadian Gemmologist, Vol.III, No.4, Spring, 1982) I, too, am sometimes found as a green stone, though you probably see me more often as red brown, white, blue, or golden colors. My R.I can vary considerably, as does my S.G. Usually you see me as tetragonal crystals, but if I am found as rolled pebbles, I am more nearly amorphous. Sometimes I show easily recognized absorption spectra, sometimes none at all. What am I?
Answer: Zircon