How To Identify and Select Crystals Part Three: Optical Effects


So far in this series we have explored how color can help us identify and select healing crystals and how we can use transparency to help with identification. Next we will look at the phenomenal optical effects that naturally occur in some of our favorite crystals! The internal structure of the crystal and the mineral inclusions within it determine how it interacts or interferes with light, causing light to scatter, reflect, refract, absorb or transmit. Understanding these common optical effects can help us identify and select crystals and increase our Crystal IQ!

Adularescence

The word adularescence comes from the word adularia, the original name for Moonstone! The optical effect of Moonstone resembles a wave of light floating like a cloud in the sky on a sunny day. It appears as if crystal is glowing from inside. As you move the crystal around, the glowing light moves too. Because the glowing light resembles the lunar light of the moon, the name adularia was changed to Moonstone!

Adularescence is caused when light interferes with the different layers and minerals the crystal is composed of. Moonstone is the most popular and well known of the crystals that display adularescence, but there are other crystals that have been known to demonstrate this optical effect too. Opal commonly demonstrates adularescence, but most people refer to it as opalescence. Other than Moonstone and Opal, Rose Quartz can demonstrate adularescence and even some Agates.

Aventurescence

Aventurescence is an optical effect that causes a crystal to look like it is sparkling. The most common crystal to demonstrate aventurescence is Aventurine, whom this effect was named after. If you look closely at a a piece of Aventurine you can see tiny flecks that look like glitter inside the crystal that make it sparkle in the light.

Aventurine is actually a variety of Quartz with other mineral inclusions. These small reflective particles can be abundant enough to change the color of the crystal. The most common color of Aventurine is green. The green color is caused by inclusions of Fuchsite, a chromium rich green crystal in the Mica family. Other colors of Aventurine like blue, gray or white are caused by inclusions of Muscovite or Ilmenite. While Hematite or Goethite can produce Aventurine in pink, orange, red and brown.

Another crystal that demonstrates Aventurescence is Sunstone. Sunstone is not in the Quartz family like Aventurine, but part of the Feldspar Group which includes Labradorite and Moonstone. Sunstone demonstrates aventurescence and gets its color from inclusions just as Aventurine does. Common inclusions that cause its color and optical effect include Copper, Hematite and Goethite.

Iridescence

This optical phenomenon is a multicolored effect caused by light diffraction. As white light passes through layers, very small slits or pores of a crystal composed of materials with different refractive indexes a prism effect causes light to separate into these amazing spectral colors.

Fire Agate, like the one pictured above, gets its iridescence from a thin layer of Limonite over its botryoidal surface. (Botryoidal is a crystal habit that makes the crystal look like a bunch of grapes) Besides Fire Agate, Fossilized Ammonite mollusks can display iridescence if they have inclusions of Aragonite layering their shell and are then called Ammolite. Rainbow Obsidian also displays Iridescence due to inclusions that diffract light. When crystals have internal fractures, such as those found in Iris Quartz (Iris Quartz is actually just a Quartz crystal that has internal fractures!) and Smoky Quartz, they too can display a rainbow iridescence.

Labradorescence

Labradorescence is a form of Iridescence found in Labradorite. Labradorites are formed by repetitive twinning layers which causes both diffraction and interference of light as it passes through the stone and reflects off its parallel surfaces. I've always referred to Labradorite as the stone of hidden beauty because its iridescent effects are only visible at certain angles. I love when someone picks one up from my collection thinking its just a boring grayish black rock only to be mesmerized by its flash when the light hits it. Below are a few different pieces from my collection that demonstrate this remarkable phenomenon in blues, greens, yellows and golds, but purple and pink can also be displayed.

Chatoyancy

Simple Chatoyancy

Chatoyancy is an optical effect that causes a luminous streak of light that runs perpendicular to the crystal. The most common crystal to demonstrate this effect is Tigers Eye. A well polished Tigers Eye appears as if it was created out of a spool of silk with a beautiful golden sheen reflected by light. When you move the crystal around the colorful sheen can disappear causing the crystal to go from light to dark in color.

You may be surprised to learn that Tigers Eye is in the Quartz Family of minerals. The chatoyant effect we see in Tigers Eye is caused by parallel, microscopic, needle-like inclusions within the Quartz. Besides Tigers Eye, Hawks Eye, Pietrisite, Seraphinite and Charoite can also demonstrate chatoyancy. Of those, only Hawks Eye and Pietrisite are in the Quartz Family as other mineral families can have inclusions within the crystal to cause this effect.

Cat's Eye Chatoyancy

Cat's Eye is an optical effect used to describe a crystal that demonstrates similar Chatoyancy as described above, but with the distinct appearance of a cat's eye. When light is reflected from the parallel, needle-like inclusions in the crystal, a band of light can be seen running through the middle of the crystal creating the appearance of a cat's eye. Some Cat's Eye crystals also have what is known as "the milk and honey effect." This occurs when light is concentrated laterally on one side of the crystal resulting in the appearance of a dark and light side.

Chrysoberyl cat's eye is by far the most famous of gems, but this remarkable phenomenon can also occur with varieties of Quartz, Tourmaline, Apatite, Beryl and Moonstone.

Asterism Chatoyancy

Asterism Chatoyancy is similar to Cat's Eye, but instead of one parallel line of light shining through the crystal there are several. If the intersecting axis points have perfectly aligned inclusions the result is a unique reflection of either a 4 or 6 ray star.

Pleochroism

Pleochroism occurs when a crystal or gemstone displays different colors or depths of colors when viewed from different angles. This occurs when light rays are absorbed differently by double refractive crystals. This effect cannot occur in singly refractive gemstones like garnet or diamond. Notice in the image below of the Andalusite faceted gemstone (photo courtesy of Creative Commons) is both orange and yellow. Pleochroic crystals can be dichroic, displaying two distinct colors, or trichroic, displaying three different colors.

The most common crystals to demonstrate pleochroism are Andalusite, Iolite, Kyanite, Kunzite and Tanzanite. I highly recommend a google image search to see some of the most beautiful images demonstrating this effect!

Conclusion

This concludes this week's post on Optical Effects! I hope you found the information helpful and easy to understand. I know how confusing some of the scientific aspects of crystals can be. I hope by breaking things down in smaller parts it becomes easier for everyone to understand and is not too boring. This particular topic served as a reminder for me of how absolutely amazing crystals are. They, in their natural form, can display such stunning flashes of color and phenomenal effects! I cannot wait to add new beauties to my personal collection so I can show off this natural phenomenon to others!

Here is an image for your Pinterest board or to save to your photos in case you wish to reference the different optical effects in the future! Until next time!

Works Cited

"Category:Images." Category:Images - Wikimedia Commons. N.p., n.d. Web.

Farndon, John, and Alec Livingstone. The Practical Encyclopedia of Rocks and Minerals: How to Find, Identify and Collect the World's Most Fascinating Specimens, Featuring over 800 Colour Photographs and Artworks. N.p.: Lorenz, 2008. Print.

"Gemstone Phenomena: Remarkable Optical Illusions of Colored Stones." Buy Gemstones: Semi Precious & Precious Stones, Natural Gems at Wholesale; GemSelect. N.p., n.d. Web.

"Geology and Earth Science News and Information." Geology. N.p., n.d. Web.

Guastoni, Alessandro, and Roberto Appiani. Minerals. N.p.: Firefly, 2005. Print.

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