Interstitial Fe3+ impurity in the growth-loci of the terminal major rhombohedral faces of
quartz protects these regions from becoming smoky-colored if the iron content is in large
excess of the Al3+ impurity content. The latter is reponsible for the trapped-hole centers
causing the smoky color, in the presence of ionizing radiation. GrowthJoci of terminal minor
rhombohedral faces may become smoky even though large excess of substitutional Fe3*
compared to Al3* impurity, is present