FMF - Friends of Minerals Forum, discussion and message board

[Matt_Zukowski]

I was examining a tourmaline in front of my computer monitor and noticed colors that I had not seen before in this piece. Large areas of the xtal were colored a bright saturated violet. I turned to look through the xtal at a compact fluorescent light source (warm), and the areas that were previously violet were now a brownish-salmon-pink color. I then decided to photograph the piece, and under my photography lights (5000k fluorescent), the areas were now a salmon pink without any brownishness. This all prompted me to google “color change tourmaline” where I found out that color change in tourmaline is not uncommon. I hope you find this and the pictures interesting.

[Matt_Zukowski]

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[alfredo]

Matt, this *might* perhaps be due to "color change" under different light sources, as in alexandrites, etc., but keep in mind too that tourmaline can be strongly dichroic and your monitor may be emitting polarized light, so you could be observing dichroism effects.

[Jesse Fisher]

The light coming from a computer monitor is often polarized. Tourmaline being highly dichroic will often show a pronounced color change depending on the crystals' orientation to polarized light. To see a good example of this, take a tourmaline crystal and observe the colors in daylight while wearing a pair of polarizing sunglasses. The color saturation will often change dramatically as you rotate the crystal perpendicular to the C axis. I've found the effect can be quite strong with California tourmalines, particularly from the Himalaya mine, but if sometimes weak in Afghani crystals.

[Matt_Zukowski]

Alfredo & Jesse – you totally nailed it – the light from my monitor must be polarized. I did not think to rotate the piece in front of my monitor. Follows are two pictures of the piece rotated 90 degrees in front of my monitor. Note that the color change occurs across the xtal – the lower part shows the brownish-salmon-pink to violet change noted earlier. I can now also see that the area under the pyramidal termination turns light green/blue when the xtal is horizontal.

Very cool – and thanks always for your replies. You guys are FMF assets!

[Mark Ost]

Almost all monitors are polarized. The quick way to tell is get a pair of polarized sunglasses and rotate them in front of the monitor. Sunglasses are designed to admit one directional vibration of light. As you rotate them in front of a polarized light source the light should extinguish at a 90 degree angle (cross polars). Nice experiment with your tourmaline.

If you really want a head scratcher; get a second polarized lens and insert it between the screen and the first polarized lens at extinction angle. The second polarized lens (the one in the middle) will "unlock" and allow light to pass through. How it does it I have no idea!