My cats like to participate in most of my activities. Their
"assistance" generally involves batting supplies off my desk. When
they sit and stare at me, it’s easy to see how one of my favorite
cat’s-eye chrysoberyl, got its name. The stone is the
same golden-honey color as my cats’ eyes, and the long, narrow band
of light across the middle of it looks like their narrowed pupils.
Cat’s-eyes and star-stones are popular names for the special effects
caused by light reflections in certain They can usually
only be seen when the gemstone is cut in a rounded or domed shape
(cabochon). The reflections that cause cat’s-eyes and star-stones
are one way in which color is displayed in
Color in as it does in all other substances, starts with
light. Visible light, the light that humans see with the naked eye,
is only a small part of the total light available. Visible light is
also called “white light,” because it looks white to us. All light
consists of wavelengths of individual colors. Visible light is a
combination of color wavelengths that blend to form white. You can
easily see all the colors in white light when moisture in the air
separates sunlight into a rainbow after a rain.
All solids, including separate the colors in light.
Nearly all substances absorb some color wavelengths. The colors that
they do not absorb are reflected back. Those reflections are the
colors that we see. Each gemstone will absorb or reflect different
color wavelengths depending primarily on the chemicals it contains.
For example, peridot absorbs all but the green color wavelengths.
The green is reflected back, making peridot look green to us. Rubies
absorb everything except red and some blue wavelengths. The red and
unabsorbed blue are reflected back to us as a deep red color.
Gemstones appear black when they absorb all color. Some gemstones in
their chemically pure state don’t absorb any color wavelengths.
Beryl is one example. Because all the colors are reflected back, the
gemstone is the same color as the light striking it. That makes it
appear colorless to us. However, tiny amounts of impurities in a
stone can cause some light to be absorbed rather than reflected.
Depending on the impurity in beryl, we see green (emerald), blue
(aquamarine), yellow (heliodor or golden beryl), or pink
(morganite).
While impurities can change the color of the whole gemstone, as with
the beryls, inclusions change only a part of a stone’s appearance.
Inclusions are relatively large amounts of some foreign substance
embedded in the gemstone. Because the inclusion has a different
chemical composition than the gemstone itself, it absorbs and
reflects different color wavelengths than the gemstone does.
Although most inclusions are undesirable, some can create effects
like chatoyancy (cat’s-eyes) and asterism (star- stones).
Chatoyancy, which comes from the French words for cat and eye, is a
band of light across a gemstone. It is not on the surface of the
stone, but comes from within and looks exactly like a cat’s eye. The
inclusions that typically cause chatoyancy are tiny, needle-shaped
crystals of rutile. Rutile is a colorless mineral that reflects
light the way diamonds do. To reflect a good cat’s-eye, the crystals
must be lined up parallel to each other. Cat’s eyes occur frequently
in chrysoberyl, but are also found in tourmaline, rubies, sapphires,
garnets, spinel, and quartz.
If the inclusions are lined up in bands in more than one direction,
with the bands all crossing one another at a middle point, the
reflected light forms a star. Stars usually have either four or six
rays. Stars, or asterism, are found primarily in rubies and
sapphires. Quartz and garnet may also sometimes have stars. Random
patterns of crystal inclusions may reflect spangles or points of
light.
Tiger’s-eye starts as the mineral crocidolite, which contains long,
asbestos-like fibers. Over time, nature replaces the fibers with
quartz crystals and the color gradually changes from blue to yellow
and brown stripes. Tiger’s-eye gets its name from the chatoyancy
visible when it is properly cut.
Color is what makes many gemstones valuable, and light is the source
of that color. Chatoyancy (cat’s-eyes) and asterism (star-stones)
are special reflections of light that add to the beauty and pleasure
of the gemstones we prize
****Sandra I. Smith, Writer ****