In unheated sapphires, transparent zircon inclusions displayed
euhedral slightly elongated forms and clear interfaces with their corundum host. Most were
confined within the host under relatively high pressures (up to 27 kbar), and showed evidence
of natural radiation-related damage (metamictization). Subsolidus reactions (i.e., the decomposition
of zircon into its component oxides without melting) of some zircon inclusions started
at temperatures as low as 1400°C, as evidenced by the formation of baddeleyite (ZrO2)
and a SiO2-rich phase. Differences in the degree of preexisting radiation damage are the most
likely cause for the decomposition reactions at such relatively low temperatures. Melting of
zircon and dissolution of the surrounding sapphire occurred in all samples at 1600°C and
above. This resulted in the formation of both baddeleyite and a quenched glass rich in Al2O3
and SiO2. From these data and observations, a systematic sequence of both modification and
destruction of zircon inclusions with increasing temperature was compiled. This zircon alteration
sequence may be used (1) as a gemological aid in determining whether a zircon-bearing
ruby/sapphire has been heated, and (2) to provide an estimate of the heating temperature.