Guinea Bissau Jewellery
By Aamir Mannan. ZIRCON (URANIUM)
Green zarcons can always be relied upon to show absorption bands, but the type of spectrum seen varies enormously with the degree of internal breakdown (metamictisation) of the zircon crystal concerned.Most Ceylon green or greenish zircons show
a strong ten-or twelve-band spectrum,with the strongest band in the red at 6535A and the other bands distributed throughout the spectrum as described under 'yellow zircons'.But even in these it will be noticed that the edges of the
bands are 'wolly', and not sharply defined as in the fully crystaline 'high type' zircons.Such stones have usually a rather cloudly or sleepy appearance.In the really 'low type' zircons with S.G. 4 or
under and no appreciable birefringence there may remain only a diffuse and rather broad band near 6530 A.In some low types, however, a narrow band at 5200A in the green is very noticeable.After heating at 800 C these and other metamict zircons having a S.G. below 4.02 develop a strong anomalous series of absorption bands at 6870,6685,6525,5890,5740,5605,4735, and 45Io A.This spectrum is rarelly met with in zircons as they occur in nature.The anomalous spectrum is shown in figure I5.I,together with the normal zircon spectrum for comparison.
a strong ten-or twelve-band spectrum,with the strongest band in the red at 6535A and the other bands distributed throughout the spectrum as described under 'yellow zircons'.But even in these it will be noticed that the edges of the
bands are 'wolly', and not sharply defined as in the fully crystaline 'high type' zircons.Such stones have usually a rather cloudly or sleepy appearance.In the really 'low type' zircons with S.G. 4 or
under and no appreciable birefringence there may remain only a diffuse and rather broad band near 6530 A.In some low types, however, a narrow band at 5200A in the green is very noticeable.After heating at 800 C these and other metamict zircons having a S.G. below 4.02 develop a strong anomalous series of absorption bands at 6870,6685,6525,5890,5740,5605,4735, and 45Io A.This spectrum is rarelly met with in zircons as they occur in nature.The anomalous spectrum is shown in figure I5.I,together with the normal zircon spectrum for comparison. 
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