Iridescent Rocks and Fossils 

Rainbows aren't just confined to the sky -- they can happen right here on Earth too! When you tilt a rock or fossil in the light and see a colorful array dancing across its surface, you are experiencing the amazing optical property of iridescence. 

 

Iridescence

What is Iridescence?

This fascinating optical phenomenon occurs when an object appears to change color depending on the angle of observation or illumination. Often seen as a shimmering rainbow effect, it can appear in soap bubbles, seashell nacre, minerals like Labradorite, butterfly wings and oil slicks. 

Iridescence occurs as a result of either internal interference or surface-level interference. 

 

Internal Iridescence

Internal iridescence is caused by light diffracted (bent and spread) and reflected from closely spaced structures inside a specimen. These can include fractures, cleavage planes, lamellae (thin plate-like structures), or tiny foreign inclusions inside a rock, mineral or fossil shell (such as Ammonites, Ammolite or Baculites).

For example, some specimens of Labradorite (a plagioclase feldspar) show a stunning range of colors from blue to green and gold when tilted in the light, thanks to the alternating thickness of internal lamellar pairs. 

Labradorescence - What is Labradorescence?

Internal iridescence in feldspars is also called labradorescence or "schiller". It is the result of light being scattered by extremely fine  exsolution lamellae, which are often less than 1/10 of a micron thick. This occurs when a super-thin layer of one mineral separates and nests inside another. 

Its lamellar structure is caused by separation during the very slow cooling of the rock. As this slow cooling happens, Calcium, Sodium, Silicon, and Aluminum ions diffuse through the plagioclase to produce distinct, alternating layers.

While not all Labradorite exhibits labradorescence -- some may have cooled too quickly or lack the correct composition -- rare, high-quality specimens called Spectrolite can exhibit a vivid, full rainbow of colors.

Surface Iridescence

Surface iridescence is similar to the optical effect produced by soap bubbles or thin films of oil on water. It is caused by the interference of light as it reflects off ultra-thin surface films produced by oxidation or alteration. 

In the mineral world, surface iridescence is most often seen on metallic minerals such as Pyrite, Hematite, Goethite, Bornite and Sphalerite. 

 

 

Conclusion

This fun optical phenomenon shows up plenty of places in our world, but perhaps most impressively in the rocks, minerals and fossils we love. Ready to add one of these geologic rainbows to your own collection? Shop here!