Gem library
Cat's eye
The chrysoberyl with a slit of light
When gemmology speaks of cat's eye with no further qualification, it means chrysoberyl and chrysoberyl alone. Every other chatoyant stone must state its name, and that is a rule of nomenclature, not a commercial preference.
- Hardness (Mohs)
- 8.5
- Density
- 3.70 – 3.78
- Crystal system
- Orthorhombic
- Refractive index
- 1.744 – 1.755
- Cleavage
- Distinct on {110}
Mineralogical identity
- Family
- Oxides, chrysoberyl group
- Species
- Chrysoberyl
- Variety
- Cymophane
- Chemical formula
- BeAl₂O₄
- Crystal system
- Orthorhombic
- Classification
- Precious stone
- Rarity
- Rare
- Indicative price
- €100 – 3,000/ct (fine honey-coloured cat's eye: far beyond)
Etymology
Cymophane comes from the Greek kyma, «wave», and phainein, «to appear»: the light seems to undulate beneath the surface. The common name describes the vertical slit of a feline eye.
Physical & optical properties
- Hardness (Mohs)
- 8.5
- Density
- 3.70 – 3.78
- Refractive index
- 1.744 – 1.755
- Birefringence
- 0.008 – 0.010
- Cleavage
- Distinct on {110}
- Fracture
- Conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Translucent to opaque
- UV fluorescence
- None
- Pleochroism
- Weak
- Colours
- honey yellow, yellow green, brown, golden
Formation, geology & history
A privilege of nomenclature
Dozens of minerals show chatoyancy: quartz, tourmaline, apatite, diopside, aquamarine. Gemmological usage nonetheless holds that «cat's eye», used alone, means cymophane, the chatoyant variety of chrysoberyl. All the others must be named in full: cat's eye quartz, cat's eye tourmaline. The convention is old, codified by the laboratories, and it has a reason: chrysoberyl gives the sharpest and liveliest effect in the whole mineral kingdom, through the fineness of its inclusions and its high refractive index.
The silk, and how it is cut
The effect arises from millions of parallel channels and needles, often rutile, aligned along one axis of the crystal. Light striking them is reflected in a narrow line at right angles to their direction. To bring it out, the lapidary must cut a cabochon whose base is strictly parallel to the needles and whose dome is good and high: too flat, and the line widens and dilutes. On a top-quality stone the slit is so fine and so vivid that bringing two light sources near makes it open and close like a pupil. This is called the milk-and-honey effect: under grazing light one side of the stone looks milky, the other golden, and the line separates them.
A family with three faces
Chrysoberyl comes in three very different forms. Ordinary chrysoberyl is transparent, yellowish green, faceted, and stays discreet on the market. Alexandrite, charged with chromium, changes colour from green to red depending on the light, and is the dearest of the three. Cymophane, charged with inclusions, gives cat's eye. All three share a hardness of 8.5, which puts chrysoberyl just behind corundum and diamond: this is a stone you wear without thinking about it, which few gems at this price allow.
How to recognise it
- The line is sharp and centredOn a true chrysoberyl the slit is fine, straight, vivid, and crosses the whole dome. A wide, blurred or off-centre band means another species or a poor orientation.
- The milk-and-honey effectLight it from the side: half the stone should lighten and the other stay golden, with the line as the boundary. No cat's eye quartz does this so frankly.
- Hardness and densityHardness 8.5, it scratches quartz and even topaz. Density 3.7, well above that of chatoyant quartz (2.65): in the hand the difference is clear.
Imitations, treatments & confusions
- Cat's eye quartz sold without the qualifierChatoyant quartz is a real stone, but it is worth a thousand times less. Omitting the word «quartz» is the commonest deception in this market.
- Fibre glass (glass cat's eye)A rod of optical fibres cut and polished. The effect is too perfect, the stone is warm, and under a loupe you see the ends of the fibres in a honeycomb on the side.
- Cat's eye apatite or diopsideAuthentic and pretty, but soft (5 and 6) and far cheaper. A hardness test on the pavilion settles it at once.
Deposits & origins
Sri Lanka (the historic reference deposit), Brazil (Minas Gerais, Espírito Santo), India, Madagascar, Tanzania, Burma, Zimbabwe
Care & precautions
- Warm soapy water and a soft brush: at hardness 8.5 and with no perfect cleavage, this is a robust stone.
- Ultrasonics are acceptable on a sound stone, to be avoided if it shows fractures reaching the surface.
- Fears neither light nor ordinary heat.
- Store separately despite its hardness: it scratches everything else, sapphire excepted.
Traditions & crystal healing
- Chakras
- Solar plexus · third eye
- Zodiac signs
- Leo · Capricorn
- Element
- Earth
A stone of protection and vigilance throughout South Asia, very present in Indian jewellery where it is one of the nine gems of the navaratna. It is said to guard its wearer from what they do not see coming. Cultural heritage, never medicine.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is my cat's eye valuable?
It all depends on the species. A cat's eye chrysoberyl runs to hundreds of euros a carat, a cat's eye quartz to euros. Always ask for the stone's full name.
Why does the line split in two?
That is the sought-after effect, obtained with two light sources: the two lines separate and then rejoin as the lamps are brought together. A good indicator of quality.
Chrysoberyl and beryl, same family?
No, despite the name. Beryl is a silicate (emerald, aquamarine), chrysoberyl is an oxide of aluminium and beryllium. They share only the element beryllium.
Tools for this stone
Cat's eye: only the tools this stone actually concerns, each carrying the value read on this page.