Figure 1: Emission spectrum of SW UV light. Figure 2: Emission spectrum of LW UV light.

Williams cautions that its existence (in addition to the SW reference line at 254 nm) “could lead to what is often interpreted as the stone “weakly fluorescing in SW”, when in fact it is probably induced by the leaking LW component”.

Using two synthetic yellow diamonds of different production (Figure 4) we were able to detect an interesting fluorescent reaction which was not present when using the classical SW/LW UV light source.

Figure 4: Synthetic diamonds.

 

We also observed strong fluorescence in different garnet species previously thought inert under both SW and LW light.

However, slight variations in energy levels (switching amongst similar LEDs) significantly affect the luminescent response. Hoover-Williams (2005) in “Crossed Filters Revisited” caution that different illuminants do not necessarily “increase the value of fluorescence in diagnostic testing.”

Conclusions and Discussion

Deep UV-LEDs are being developed and a 270 nm UV LED is now available for under $ 100. Fox UV LEDs emit a narrow band focused on the i-line of mercury (365 nm) and could be introduced as a new reference standard for LW fluorescence reactions now.

A set of UV-LED “masters” at 254 nm (when commercially available), 270 nm, 365 nm and 395nm (all available) could be developed for gem testing in the future.

A stone might then be described as “no fluorescence at 254nm, slightly yellowish at 365nm, strong greenish yellow at 395 nm”. The use of a UV-VIS spectrometer could help to compile an accurate spectro-fluorometry data-base for many gemstones.