Is my rock really fluorescent?

by | Hobby

A question often asked: “Is the blue fluorescence in this rock real?”. This is a pretty common question, or often a wrong conclusion made by many first starting out in our hobby.

Our UV lights have UV bandpass filters (“filter” defined here for our purposes as UV transmitting, visible blocking) which block the visible light emitted by the UV lamps. Longwave mineral lights usually have a longwave filter (Wood’s Glass or equivalent) while shortwave mineral lights will have a Hoya U-325c filter. These filters do a great job of blocking the bright bluish light allowing us to observe mineral fluorescence (see the UV-Topics page for more on how this works).

But they are not perfect; all filters let some blue/purple visible light escape. This bluish light is what causes cameras to make everything look blue if the picture is not taken properly. It also makes white (or light colored) minerals that are not fluorescent look like they fluoresce a blue/purple color. Even the best of us can be fooled by this – I call it “blue bleed”, a reflection of the visible light output by the mineral lamp. It can be very misleading.

So how do you verify if your mineral is actually fluorescing? Most are pretty obvious: willemite, scheelite, tugtupite, etc – no need for verification. They are brightly fluorescent and never in doubt. But I immediately suspect “blue bleed” when someone shows me a white mineral that is fluorescent a dull blue/purple. Some, of course, really are fluorescent (like many feldspars) but most are not. You can easily verify fluorescence under shortwave UV using a simple piece of glass. Shortwave UV (UVC) is blocked 100% by ordinary glass. But the blue/purple light leaking out of our lamps is not affected. just stick a piece of glass between the lamp and the specimen. Does the rock still look like it is fluorescing? That’s “blue bleed”, not fluorescence. But if the color changes (or goes away) when the glass is inserted in the path you likely have fluorescence. (Side note: you might notice that one side of the glass is fluorescent under shortwave. Turn it around and it won’t fluoresce. This fluorescence is caused by the manufacturing process of float glass – the side contacting the tin float will fluoresce).

Longwave is a little more complicated. UVA (LW) is passed by glass. To perform the test above you need something special. OP3 completely blocks all UV and can be used for both UVA and UVC. Alternatively, a piece of solar window film would probably work.


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