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Cat's-Eye Opal
Pictures of Cat's-Eye Opal
Author: Hobart M. King, PhD, GIA Graduate Gemologist
What is Cat's-Eye Opal?
Cat's-eye opal is a phenomenal opal that exhibits chatoyance when properly cut into a cabochon. The chatoyance is a bright line of light across the surface of the stone caused by reflections from tiny parallel mineral needles or hollow tubes that span the width of the gem.
Table of Contents
What is Cat's-Eye Opal? Another Variety of "Cat's-Eye" Cutting a Cat's-Eye Opal Sources of Cat's-Eye Opal |
Many other gems exhibit chatoyance; the best-known are chrysoberyl and tiger's-eye. Specimens of chrysoberyl exhibit the finest chatoyance, and tiger's-eye is the chatoyant gem most widely used in jewelry. Chatoyant opal is much more rare. The most common eye-producing mineral is rutile.
When viewed under a source of incident light, the "eye" will respond in three different ways. It will move across the surface of the stone: 1) if the stone is moved, 2) if the light source is moved, or 3) if the head of the observer is moved. This "moving eye" is what makes chatoyant gems so interesting.
Another Variety of "Cat's-Eye"
An unusual variety of cat's-eye has been seen in precious opal found near Spencer, Idaho. The feature within this opal that produces the cat's-eye is thought to be "faults" or "misalignments" of the minute silica spheres, whose orderly arrangement is responsible for the play-of-color phenomenon. [1]
This variety of cat's-eye was first reported by J.V. Sanders, who also found asterism in opals from the same deposit. Previously, J.V. Sanders was the first person to convincingly explain the play-of-color phenomenon in opal. [2]
Cutting a Cat's-Eye Opal
A special type of rough is required to cut an opal cabochon that exhibits a cat's-eye. First the rough must contain a large number of thin reflective mineral needles or hollow tubes with a parallel orientation.
Then the rough should be prepared so that the mineral needles will be parallel to the base of the cabochon. The cabochon is then cut into a circular shape or into an oval with the needles oriented parallel to the short axis of the oval. The dome of the cabochon is then cut and polished. This will produce a round cabochon with an eye, or an oval cabochon with an eye that bisects the stone into two equal parts along its longest dimension.
Sources of Cat's-Eye Opal
Cat's-eye opal is a rare and unusual find. We always look for interesting opals when we visit the Tucson Gem and Mineral Show and while we are viewing gems on the internet. We have only seen cat's-eye opal a few times, and usually the only locality information available for the material is "Africa". We purchased a few cabs from GemSelect that were cut from rough found in Madagascar.
We have seen beautiful semitransparent orange and orangy yellow cabs cut from rough found in Tanzania offered for sale by AJS Gems. Mindat lists one locality for cat's-eye opal in Western Australia.
Much of the material that we have seen offered for sale as "cat's-eye opal" has a "brecciated" appearance that causes deviations in the line of the eye. It isn't pretty. We have also seen opal from Idaho labeled "cat's-eye opal", but there were no obvious mineral crystals or tubes reflecting the light, with a weak eye that is only visible across a portion of the cab.
Cat's-Eye Opal Information |
[1] The Structure of Star Opals, by J.V. Sanders, Acta Crystallographica, Volume A32, pages 334–338, 1976.
[2] Gemstones of North America, by John Sinkankas, Volume III, Geoscience Press, Inc., 1997. |
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