Chrysoberyl is a beryllium aluminum oxide and one of the hardest gem minerals, at 8.5 on the Mohs scale. It produces three main gem types: yellow to green faceted chrysoberyl, cat's eye chrysoberyl (also called cymophane) and the color-change variety alexandrite. Its hardness, toughness and bright luster make it an excellent jewelry stone. Fine cat's eye chrysoberyl from Sri Lanka has been prized in Asia for centuries.
What chrysoberyl is
Despite the name, chrysoberyl is not a type of beryl. Both contain beryllium, but beryl is a silicate and chrysoberyl is an oxide with a different crystal structure. The name comes from the Greek chrysos, "golden," and beryllos, a name for beryl and other green stones.
Chrysoberyl crystallizes in the orthorhombic system. Crystals are often flat and tabular, and they commonly twin in groups of three. These cyclic twins form a pseudo-hexagonal shape that looks like a six-sided star or wheel, sometimes called a trilling. The page on crystal forms and twinning explains how such twins develop.
Chrysoberyl forms mainly in granite pegmatites and in mica schists where beryllium-rich fluids met aluminum-rich rocks. Because it is hard and dense, it survives weathering and collects in river gravels. The gem gravels of Sri Lanka are the classic example.
Yellow and green chrysoberyl
Ordinary chrysoberyl ranges from greenish yellow through golden yellow to yellowish green and brown. Iron is the usual coloring agent. Bright lemon-yellow and greenish-yellow stones were popular in Victorian and Georgian jewelry, sometimes under the name "chrysolite," a term that was also used loosely for peridot and other stones.
Faceted chrysoberyl is lively and bright. Its refractive index of about 1.75 is higher than that of most colored gems, so a well-cut stone has strong brilliance. Good yellow chrysoberyl is less common in jewelry today, but collectors seek it as a durable, often affordable alternative to yellow sapphire.
Cat's eye chrysoberyl
When people say "cat's eye" without naming a gem, they mean chrysoberyl. Other minerals can show the effect, but by trade convention only cat's eye chrysoberyl may be called simply "cat's eye." Other gems need the mineral name, as in cat's eye tourmaline or cat's eye apatite.
How the eye forms
The effect is called chatoyancy, from the French chat, "cat." It comes from many fine, parallel needle-like inclusions or hollow tubes inside the stone. When the stone is cut as a domed cabochon with its base parallel to these fibers, light reflects off them as a single bright line across the dome. As the stone or the light moves, the line glides across the surface. Our page on gemstone optics compares chatoyancy with asterism and other phenomena.
Milk and honey
In the best cat's eyes, the side of the stone nearest a light source looks milky while the other side keeps its body color. This "milk and honey" effect is a sign of fine material. Top stones have:
- A sharp, narrow, straight eye centered on the dome.
- An eye that opens and closes cleanly when the stone is rotated under a light.
- A warm honey-yellow to golden-brown or greenish body color.
- A smooth, well-proportioned dome.
Cat's eye chrysoberyl is closely tied to Sri Lanka, covered on our page about the gems of Sri Lanka. India, Brazil, Madagascar and Tanzania also produce it. Some cat's eyes are irradiated to deepen their color, and this should be disclosed.
Cat's eye chrysoberyl has also been called cymophane, from Greek words meaning "wave" and "appearing." In older use, cymophane sometimes meant any chatoyant or milky chrysoberyl.
Alexandrite
Alexandrite is the color-change variety of chrysoberyl. Traces of chromium give it green to bluish green color in daylight and red to purplish red under incandescent light. It was first found in the Ural Mountains of Russia in the 1830s and named for the future Tsar Alexander II. Later finds came from Sri Lanka, Brazil, India and East Africa.
Cat's eye alexandrite combines both effects: a sharp eye and a color change. It is one of the rarest varieties of chrysoberyl. Alexandrite shares the June birthstone place with pearl and moonstone, listed on birthstones by month. The full story of the gem is on its own page.
Identifying chrysoberyl
Chrysoberyl's combination of properties narrows things down quickly:
- A refractive index around 1.746 to 1.755, with moderate birefringence.
- A specific gravity of about 3.73, higher than that of quartz, tourmaline or beryl.
- An iron absorption band in the blue-violet, visible in a spectroscope for yellow and green stones.
- Fine silk and fingerprint-like inclusions under magnification.
Look-alikes for cat's eye include chatoyant quartz, tourmaline, apatite and glass made with parallel fibers ("fiber optic glass"). Glass shows a honeycomb pattern from the side under magnification. Synthetic chrysoberyl and synthetic alexandrite exist, mostly grown for alexandrite. The gemological instruments page covers the tools that separate them.
Durability and care
Chrysoberyl is one of the most durable gems after diamond and corundum. At 8.5 on the Mohs scale, it resists scratching from dust and everyday contact. It has only weak cleavage and is tough, so it handles daily wear in rings. It is stable to light and normal heat. Our page on gemstone durability compares it with other jewelry stones.
Clean chrysoberyl with warm soapy water and a soft brush. Ultrasonic and steam cleaning are generally safe for untreated stones without fractures.
Value factors
For faceted yellow and green chrysoberyl, bright, saturated, clean stones are the most desirable. For cat's eye, the sharpness of the eye and the milk and honey effect matter most, followed by body color and clarity. For alexandrite, the strength and quality of the color change dominate value. Size matters for all three, since large fine stones are rare.
Frequently asked questions
Is chrysoberyl the same as beryl?
No. Both contain beryllium, but chrysoberyl is an oxide and beryl is a silicate. They are different minerals with different hardness and optical properties.
What does "cat's eye" mean when no gem is named?
By trade convention, "cat's eye" on its own refers to cat's eye chrysoberyl. Other chatoyant gems should carry their mineral name, such as cat's eye quartz.
Is alexandrite a chrysoberyl?
Yes. Alexandrite is the chromium-bearing color-change variety of chrysoberyl.
Is chrysoberyl good for an engagement ring?
Yes. At 8.5 on the Mohs scale and with good toughness, chrysoberyl is one of the better colored gems for everyday rings.