corundum, chemistry.2, gems

Harder and Schneider investigate experimentally how cooling rate controls whether Ti-bearing corundum develops asterism or a cornflower-blue color. They relate the two effects to the behavior of titanium during cooling: under conditions permitting exsolution, titanium is precipitated as oriented rutile needles, producing the silk responsible for asterism; under different cooling conditions, titanium remains incorporated in the corundum lattice, where interaction between iron and titanium produces blue coloration. Thus, corundum of essentially comparable composition can develop either the oriented rutile inclusions required for a star effect or the transparent blue appearance characteristic of sapphire, depending upon its thermal history. The paper emphasizes cooling rate as a decisive factor governing the distribution of titanium between the corundum lattice and exsolved rutile.