Why evil eye glass beads are blue ?

Blue is the easiest colour for the bead maker to obtain deliberately.In fact,variations in the temperature or other features of the furnace,such as variations in the supply of oxygen and the rate of cooling have little effect on this pigment,which generally provides the same tone.This is the easiest tint to obtain in a glass workshop as glass turns blue from light green of its own accord,given certain conditions.Cobalt oxide which is what gives a blue tint,is found in a number of structures in its natural form.However it is used,the same blue tone emerges in the end product.In other words,any modifier with cobalt in it will act as a colourant,turning the glass bead blue.Cobalt oxide appears black in its raw state ,turning blue as glass.Variations in the capacity of the glass maker's furnace,temperature,the level of oxygen in the firing,the speed of cooling and other similar factors in no way alter the tone of blue which we see in the finished bead.
In fact,bue being such an easy tone to obtain,it was a kind of tester for glass technology,and it explains,to some extent,the relationship between its use in making of glass beads and the widely held belief's concerning blue,the sky color.