Abstract:
Zircon is a common gem mineral found in gem gravel beds of Sri Lanka. Heat treatment
technique is used to enhance the colour and clarity of reddish-brown zircons. The objective of
this research was to study the properties of Heat-treated Yellow Zircons (HYZ) produced from
Sri Lankan Reddish Brown Zircons (SLRBZ). Several SLRBZ were annealed to produce yellow
colour at about 600 ◦C for 30 minutes soaking period. Five HYZs were randomly selected for
a detail study. Specific gravity of these stones was measured which had an average of 4.4,
affirms that most of them belongs to intermediate type zircon. FTIR, Raman, UV-Vis and XRF
analyses were performed for further characterization. The strong hydrous component peak at
3800 cm-1 and 3400 cm-1, weak banding at 854 cm-1 and very poor peak at 609 cm-1 in FTIR
spectrum illustrates the metamictization. This was further clarified by Raman peaks, where Full
Width Half Maximum (FWHM) of the 1008 cm-1 peak was 20. Also, the weakening of bands
between 230 cm-1 to 200 cm-1 and broaden width of the bands. But Geiger number in Counts
Per Minute (CPM), which lies between 16 to 32 (less than 100), indicates the radiation level
of HYZ is not harmful. Broad absorption band in UV region with a tail in visible region was
observed in UV-Vis of the SLRBZ and it was not observed in HYZ. So, this confirms that brown
colour tint has been reduced during the heating. Also, those HYZ were exposed to sunlight for
two weeks and observed that there was no any loss of colour. Further mass percentage of Zr, Si,
Hf, Ur and Th were examined and average of 72%, 25%, 1.16%, 0.06% and 0.02% were found
in each of these elements respectively. Tb3+ peak at 652 nm of UV-Vis and Ur5+ peak at 6668
cm-1 of FTIR was also detected. Concludingly, trace elements of Th, Ur, Hf and Tb were found
in all HYZs. And, though HYZs are metamict, the enhanced colour is stable and they are safe
to wear in jewelleries.
