Why an article about calcite? There is no calcite in granite; it’s true. But here at the Rock of Ages Visitors Center we carry a variety of rocks and minerals, not just granites or their primary constituents, quartz, feldspars and micas. We use rocks and minerals to educate school groups and visitors about geology and sell specimens in our ongoing desire to create the next generation of “rock hounds.”

Calcite is a mineral belonging to the class “carbonates” and is one of the most abundant minerals in the world, making up about 4% by weight of the earth’s crust. It appears in many forms, two of the most common being limestone and marble, or as I refer to them when picking on my friends at Vermont Marble “poor man’s granites”. Crushed calcium carbonate has many industrial applications; it is the powdery white substance that keeps your gum from sticking to the wrapper; it’s used in paints and plastics, toothpaste and cosmetics. Its ability to “capture” other materials makes it ideal as a sorbent for sulfur dioxide during the burning of fossil fuels, so there are many “green” applications for calcite. But one of its most interesting properties, birefringence, has placed it on the cutting edge of technology.

Birefringence is the commonality that links Harry Potter, the dreaded Klingon Empire and humble calcite. Birefringence or “double refraction” simply defined is a material’s ability to split incoming light into two beams, each now traveling at a different angle (the angles of refraction) and each traveling at a different rate of speed. Calcite, used as a lens in this fashion, can be used to produce a “double image”, an experiment you likely participated in during a junior-high science class. But this same property has recently been exploited by scientists to produce invisibility.

By 2006 scientists working in the field had achieved invisibility of microscopic (not visible to the unaided eye) objects with manmade materials referred to as “metamaterials.”  But late in 2010 two research teams working independently of one another announced that they had made macroscopic (visible to the naked eye) objects invisible relative to certain wavelengths of light.

Baile Zhang, an optical engineer, and his colleagues at the Singapore—MIT Alliance for Research and Technology Centre in Singapore used two pieces of calcite to create a filter that bends incoming and outgoing light rays in a desired way, tricking the eye so that the light waves appear to be bouncing off the surface of the filter and from the ground beneath it, rendering the object beneath the filter invisible. To date, Zhang and colleagues have experimented with filters that work underwater and have been able to hide a metal wedge 38 mm long and 2 mm high from red, green and blue visible light.

A second group or researches, physicist Shuang Zhang and colleagues working at the University of Birmingham, UK, and John Pendry from Imperial College London, built a calcite invisibility cloak that works in air, hiding a peppercorn and other small objects a few centimeters in height.

To date these researchers have not created anything quite as exciting as Harry Potter’s invisibility cloak or the Klingon Cloaking shield, but their accomplishments herald greater things to come. Researchers anticipate a myriad of applications, such as being able to disguise a submarine or render military installations invisible. Others have suggested that there might be many uses for the technology in cosmetics. What a great way to hide a pimple before that first date (or in my stage of life a great way to hide crow’s feet)! But the technology has its limitations right now. The object under the calcite filter is only “invisible” from certain angles and in certain frequencies of light. And the greatest limitation to date is that the crystal itself is still visible.

It might look just as awkward to have a piece of calcite hanging from your face as it would to be seen with a pimple. Or imagine hiding a munitions depot under a giant slab of calcite. You might as well just paint a bullseye directly on the roof!  But now that human engineering has fired human imagination, there just may come a time in the not-so-distant future when invisibility will be as commonplace as satellite technology is today.

To learn more about this exciting new field of human endeavor, please see the following sites:






Todd Paton has more than 20 years of experience working in the Vermont tourism industry. Currently the Director of Visitor Services for Rock of Ages, one of Vermont's oldest, continuously operating attractions, he has served on the board of directors of the Central Vermont Chamber and the Vermont Hospitality Council. He is an active member of the Vermont Chamber of Commerce and the Vermont Tourism Network. He is a past Chair and current member of the board of directors of Vermont Attractions Association, a consortium of Vermont attractions established in 1956 to promote the highest standards of hospitality among Vermont's tourism-related properties.