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December 1999

Gemstones & Pearls: Gemology

When Fool's Gold Marries Quartz

Bound Together for Eternity

Pyrite and quartz are often perfect companions. A smattering of pyrite crystals sprinkled generously through quartz often lends a subtle but entrancing metallic glitter.

When a skilled cutter positions the lustrous inclusions of pyrite so you can see the crystal faces with the naked eye, the gem can become a collector's item. You can stock gems like these as a curiosity and a sales tool to show customers the wonders of gem formation.

Here are some gemological details that may interest your more sophisticated customers.

A basic question is whether this pyrite-in-quartz gem is more correctly called pyrite or quartz. There are arguments in favor of pyrite. "There is more inclusion than gem," says Simon Watt of Mayer & Watt, Maysville, KY, who owns the gem. The profile view illustrates the point, and a specific gravity test indicates the gem is much heavier than quartz. However, refractive index indicates quartz.

Another gemological factor to consider is when the pyrite forms. "Pyrite often forms its own birth-chambers," says The Photoatlas of Inclusions in Gemstones (E.J. Gübelin and J.I. Koivula, ABC Edition, Switzerland). This suggests the pyrite inclusion forms at the same time as its host mineral, which makes it a syngenetic inclusion. Conversely, epigenetic inclusions form after the development of the host crystal.

In addition, the pyrite in this example clearly shows a typical step-like growth pattern on the surface of its crystal faces. A few spots along the division between quartz and pyrite show spectral interference colors – areas of stress created by the growth rates of the different minerals. Aside from these rather minor flares of color, the marriage of this quartz and pyrite seems permanent.

  • Mayer & Watt, Maysville, KY; (800) 344-4110.

by Robert Weldon, G.G.

Front view of the pyrite-in-quartz gemstone.
Profile view of the pyrite-in-quartz gemstone. Gem courtesy of Mayer & Watt, Maysville, KY.



Copyright © 1999 by Bond Communications.



 

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