Carved in Stone

| Published on May 7, 2011

Q is for Quartz

This versatile mineral has been used in everything from ceremonial rattles to sonar transducers to quartz watches.

By Bill Langer

Quartz (kwôrts) noun – Intensely frozen ice (Gaius Plinius Secundus, 77 A.D.)

Back when our daughter, Kimberly, was a tyke (she now has tykes of her own), she, my wife, Pam, and I did a lot of camping. One afternoon, while wandering through one of our national parks, Kimberly picked up a couple hunks of quartz and said, “Aren’t these pretty?” When I asked her why she chose those rocks, she replied, “Because they look like ice!”

Kimberly was not alone in thinking that quartz looks like ice. Roman naturalist Plinius Secundus (Pliny the Elder) called quartz ‘rock crystal’ and said it ‘is formed of moisture from the sky falling as pure snow…it is hardened by excessively intense freezing…and that it is a kind of ice is certain’ (Naturalis Historia, 77 A.D., Liber XXXVII, Capitulum 23, 26).

As time passed, people learned a bit more about rocks. In 1556, Georgius Agricola published De Re Metallic, a book about mining practices and minerals. Agricola referred to quartz as an ‘unusual stone’ and attributed its origin to ‘stone juice’ that flowed through the country rock and hardened into a mineral.

Unusual stone, indeed. You might recognize the name Pierre Curie as the man who, together with his wife, Marie, received the Nobel Prize for their work on radioactivity. But during 1880, before Pierre had even met Marie, he and his brother, Jacques, made a remarkable discovery showing just how unusual quartz is. They noticed that when the shape of a quartz crystal was altered by pressure, an electrical charge was emitted from the mineral. They also demonstrated that, conversely, if an electrical charge was applied to a quartz crystal, the crystal would change shape or vibrate. This phenomenon became known as ‘piezoelectricity’ (from the Greek piezen, to press tight).

The piezoelectric properties of quartz ultimately led to its use in a variety of applications with one of the first being sonar transducers, which were used during World War I to locate submarines. Today, one very common use is in quartz watches. In that application, a tiny piece of quartz crystal, cut in the shape of a tuning fork, receives an electrical charge from the watch battery or storage cell. The electricity makes the quartz vibrate (or oscillate) at the rate of 32,768 times per second. As the quartz oscillates, it generates electrical pulses at the same rate of 32,768 per second. The pulses are sent to an integrated circuit that counts the pulses and sends out a single electrical pulse each time the count hits 32,768. The one-second pulses activate digital displays or run a stepping motor that drives a chain of gears and the watch hands.

Quartz has one other intriguing property. When you strike a quartz crystal, or rub two crystals together, they generate light. The Uncompahgre Ute from central Colorado recognized this phenomenon. They filled ceremonial rattles made from translucent buffalo rawhide with clear quartz crystals, and when the rattles were shaken during nighttime ceremonies, the stress of the quartz crystals impacting one another produced flashes of light visible through the buffalo hide.

Kimberly, Pam, and I took the ice rocks Kimberly found back to our tent, and, after dark, we rubbed them together. Unbeknownst to us, the light we (and the Utes) witnessed was due to yet another property of quartz — ‘triboluminescence’ (from the Greek tribein, to rub, and the Latin lumen, light). When quartz is struck (as in a rattle) or rubbed together, a discharge in energy is created by a change in electron excitement level associated with the breaking of the silicon-oxygen bonds. Wow! And we thought it was just a cool light show.

Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at blanger@usgs.gov.

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