When someone says “cat’s-eye” in reference to a colored gemstone they’re referring to a variety of chrysoberyl known as cymophane, polished to display chatoyancy in a thin central pattern, reminiscent of the gaze of a watchful feline.
Cat’s-eye and its cousin, color-change chrysoberyl, are gemstones which can display phenomenal optical properties.
Derived from the Greek words “chrsyso” meaning golden, and “beryl” meaning green, chrysoberyl is not actually from the beryl family of minerals. They have similar appearance and composition, but beryl is beryllium aluminium cyclosilicate (Be3Al2Si6O18) while chrysoberyl is a beryllium-aluminum oxide mineral (BeAl2O4) and has been classified as its own independent species.
High beryllium concentrations most often occur on the margins of magma bodies during final stages of crystallization, so chrysoberyl usually forms in pegmatites and metamorphic rocks associated with pegmatites.
A robust 8.5 on the Mohs scale, chrysoberyl is also harder than beryl. In fact it’s the third-hardest common gemstone, coming in between topaz and corundum on the Mohs scale. Its hardness and high specific gravity permit it to survive in sediments where other minerals have been destroyed by weathering, abrasion and erosion over time.
Color and varieties
Most chrysoberyl is yellow to golden, with some presenting as green, brown and reddish-brown. The two varieties which can produce phenomenal optical properties are cymophane and alexandrite.
Cymophane comes from the Greek words meaning wave and appearance. This variety of chrysoberyl includes microscopic tube-like or needle-like inclusions of rutile occurring in parallel. The phenomenal optical effect called chatoyancy is produced by polishing the gemstone en cabachon, with a domed top. When planned and executed effectively the fibrous structures are parallel to the base of the gem and a streak of reflected light manifests perpendicular to the banded inclusions, as with the bright chatoyant fan seen below.
This effect would not be visible if the gemstone were faceted instead of cut en cabochon. The line of reflected light has been compared to the reflection seen across the top of a spool of silk when moved back and forth under a light source.
Other minerals such as corundum, tourmaline, spinel and quartz can be polished to produce a similar “cat’s-eye” effect, but only chrysoberyl gets referred to as “cat’s-eye” with no other designation. The others are referred to as quartz-cat’s-eyes or sapphire-cat’s-eyes (etc.) Most cymophane is found in Brazil.
Primarily found in Russia, color-change chrysoberyl was named alexandrite, after Russian Czar Alexander II. This variety substitutes chromium for aluminum in the crystal structure, which causes significant absorption of light in the yellow region of the visible spectrum. That spectral absorption causes the gemstone’s color to present differently in different ambient illumination environments.
As seen below, alexandrite appears green in daylight, where the full spectrum of visible light is present. But when the same alexandrite gemstone is placed in a lighting situation with less blue and green in its visible spectrum (incandescent lighting or candlelight) its color appearance changes from green to red.
This phenomenon is known as metamerism and is attributable to spectral absorption, rather than pleochroism which results from different viewing angles. In general, the thicker the gemstone the stronger the color-change.
Cat’s-Eye Alexandrite: Video
In rare cases where alexandrite forms with needle-like inclusions in parallel, the dual cat’s-eye and color-change qualities can be combined to stunning effect, as seen in this terrific AJS Gems video: Around 30 seconds in the ambient illumination is changed and the gemstone’s color shifts from green to red.
During the Victorian era ordinary yellow-green chrysoberyl was called “chrysolite.” That term was historically applied to a number of goldish-green to olive colored gemstone, including peridot and olivine, and is no longer used as a gemstone descriptor.
Golden, brown and reddish-brown chrysoberyl
The darker iterations of chrysoberyl include golden, brown and reddish-brown, seen here in the rough. Ordinary chrysoberyl is frequently faceted into common gemstone shapes. They are often placed in rings and pendants, as their natural hardness makes them suitable for daily wear.
Protection and clear-thinking
The practice of crystal healing is ancient. Chrysoberyl was first viewed as a talisman of protection, used to keep disaster at bay. Over time it was thought to align chakras and increase the spiritual and personal power of the wearer. Even today, crystal healing proponents suggest that chrysoberyl may promote discipline, self control and clear thinking.
Whatever the case, whether it’s the ordinary variety, Cat’s-Eye or Color-Change, a chrysoberyl gemstone is a practical and durable choice for adornment, as well as an interesting conversation piece.