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Pleochroism and Iridescence


#1

Are pleochroism and iridescence basically the same concept? Many thanks for your feedback.


#2

Yes and mostly no. Both are color changes yes. Iridescence shows with out the use of polarized light and is most often caused by inclusions , twinning plains and alternating layers of minerals . Pleochroism shows up under polarized light and is due to the crystal structure and how the light travels through it.


#3

NO, they are two different things…iridescence is due to interference colors like an oil film on water out in your driveway. Pleochroism is two or three distinct colors in a crystal or gem when viewed from different angles (along different axes). Yes, a polariscope will bring out pleochroism, but in many cases, you can see it by merely viewing the gem from different angles. Classic examples would be viewing a tourmaline along the A-B axes (one color) and along the C axis (another color). Typically a pink tourmaline will be paler along the A-B axes and more intense pink along the C axis. If it is the same shade of pink along all axes, it is not tourmaline and is probably a pink spinel, which is likely worth a lot more (sigh, ask me how I know this…) Another classic example is blue sapphire. Viewed looking straight down into the table of the stone, we see blue, the more expensive, preferred color. Looking from the side, we see green, as most blue sapphires are green on the A-B axes. Lots of other stones, such as iolite and tanzanite. In tanzanite, three colors, but when heat treated, only two. A few stones are dichroic, but the dichroism isn’t that noticeable without looking through a dichroscope, e.g., aquamarine, amethyst.
HTH, royjohn