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Cape Cod's "Magic" Quartz Pebbles

by Robert N. Oldale



When you get home, go to a dark place (such as a closet) and allow your eyes to adjust to the darkness. Now, wearing safety goggles or safety glasses to protect your eyes, firmly tap or rub the quartz pebbles together. When you do this, the quartz pebbles will glow near the site of impact or rubbing.

See Them GLOW in the Dark!

Quartz pebbles and sand from a Cape Cod Beach.
Photograph by Dann Blackwood of the
U.S. Geological Survey, Woods Hole.

On Cape Cod, we are never very far from the beach. On your next trip to the shore, collect two well rounded milky white pebbles about the size of small-to-large chicken eggs (See figure above). These pebbles, abundant on all the Cape beaches, are composed of quartz. Quartz is a hard crystalline mineral with molecules made up of one silicon and two oxygen atoms. It is one of the most important rock forming minerals and is the second most common mineral on Earth. Feldspar, the most common mineral, is a major component of granite, and is mostly composed of aluminum and silicon. Feldspar, unlike quartz, is too soft to make sand or pebbles.

Quartz makes up all but a very small part of the sand on Cape Cod beaches and was originally formed as veins filling fractures in the solid rock of New England or as a major component of granite.

When you get home, go to a dark place (such as a closet) and allow your eyes to adjust to the darkness. Now, wearing safety goggles or safety glasses to protect your eyes, firmly tap or rub the quartz pebbles together. When you do this, the quartz pebbles will glow near the site of impact or rubbing. The glow is called luminescence or, more specifically, triboluminescence.

Quartz pebbles are amazing, but they are not magic, and of course they are found in many places in addition to Cape Cod. The triboluminescence is not fully understood. One explanation suggests that electrons within the quartz are temporarily dislodged from their orbits and, when they return to their natural state, energy is released (in this case in the form of light).

Another possible explanation relates to the piezoelectric nature of quartz. Piezoelectricity is created when mechanical stress causes electrons within a crystal to flow, producing an electric current. Conversely, when a current is applied to a piezoelectric substance, such as quartz, the substance will deform and oscillate (vibrate) at a certain rate or frequency. These characteristics allow the use of quartz in crystal phonograph cartridges (if you are a kid, ask a grown-up about the phonograph) or in quartz watches. Tapping or rubbing quartz pebbles together causes a mechanical stress within the quartz. The stress, in turn, causes an electric current to be generated within the pebbles. This current may ionize gases trapped within the quartz, producing the glow in the same way that current flowing between the ground and clouds in a thunderstorm produces a lightning bolt. Of course you have only my word for all this. I hope I have made you curious, so that, regardless of your age, when you go to the beach you will collect two quartz pebbles, go to a dark place, and tap or rub them together to see if they really do glow. Remember to wear safety goggles or glasses.

A slightly different version of this article appeared in the CAPE NATURALIST, the journal of the Cape Cod Museum of Natural History, 1997, volume 24, pages 44-45."

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Related publications:
Coastal Erosion on Cape Cod: Some Questions and Answers
Geologic History of Cape Cod, Massachusetts


Reference to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. Any views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.


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