"Cat's eye" describes an effect, not a mineral. Any stone containing many fine parallel fibres or channels, cut as a cabochon with the fibres running across the dome, shows a bright line of light — the eye — that moves as the stone is turned. In the trade, and in the Navratna tradition as the stone of Ketu, cat's eye without qualification means chrysoberyl cat's eye, the finest and most valuable of the chatoyant stones. Every other chatoyant material must carry its own name: quartz cat's eye, apatite cat's eye, tourmaline cat's eye.
What chrysoberyl cat's eye is
Chrysoberyl (beryllium aluminium oxide) containing dense parallel needle inclusions, cut en cabochon. Properties: RI 1.746–1.755 (two readings, though a cabochon is read by the distant-vision method or not at all), SG 3.71–3.75, hardness 8.5, doubly refractive, typically greenish yellow to honey to brownish; the finest show the "milk and honey" effect — the stone appears half milky, half honey-coloured when lit from the side. The eye is sharp, straight, well-centred and bright.
What is sold as cat's eye
Quartz cat's eye. The commonest substitute — chatoyant quartz, usually greyish, greenish or brownish, with a softer, broader eye. RI 1.54–1.55, SG 2.65, hardness 7. Noticeably lighter in hand than chrysoberyl of the same size.
Apatite cat's eye. Yellow-green to blue-green, with a good eye. RI 1.63–1.64, SG 3.2, hardness 5 — far too soft for the wear chrysoberyl tolerates.
Tourmaline cat's eye. Green, pink or other colours, RI 1.62–1.64, SG 3.06, often a less sharp eye from coarser tubes.
Sillimanite (fibrolite) cat's eye. Greyish, silky, RI 1.66–1.68, SG 3.25.
Fibre-optic glass. Manufactured glass made of bundled fibres, producing an unnaturally sharp, straight, bright eye in any colour — including colours no natural cat's eye shows. Under magnification the fibres are perfectly regular and hexagonally packed; SG and RI are those of glass; the eye is "too good". Very common in cheap jewellery and often sold as lehsunia.
Synthetic chrysoberyl cat's eye. Exists but is uncommon; identified by growth features.
Scapolite, beryl, diopside and other chatoyant stones. Each with its own RI and SG.
How the laboratory examines a claimed lehsunia
Because the stone is a cabochon, the refractometer gives at best an approximate reading, and specific gravity becomes the primary identity test: 3.73 for chrysoberyl against 2.65 for quartz and about 2.5 for glass is decisive. The polariscope confirms double refraction (glass is singly refractive with strain). Magnification examines the fibres — natural needles versus manufactured fibre bundles — and the character of the eye. Hardness is inferred, not tested. Report states the species and, where chrysoberyl, the quality of the eye.
What makes a fine eye
Sharp, narrow, straight, centred, bright, and moving smoothly across the dome; a strong milk-and-honey effect; a well-proportioned, symmetrical cabochon with the base parallel to the fibres; body colour in the honey to greenish-yellow range preferred by most markets. These are grading judgements.
Treatments
Chrysoberyl cat's eye is rarely treated. Irradiation to darken colour has been reported and is uncommon. A report normally finds no indications of treatment.
Frequently asked questions
Is quartz cat's eye fake?
No — it is a natural chatoyant stone, sold legitimately under its own name and as a traditional substitute. It is a fake only when sold as chrysoberyl.
How can I tell fibre-optic glass?
The eye is too perfect and may show in a colour no natural stone has (bright blue, pink, red). Under a loupe the fibres are absolutely regular. It also feels light.
Why is chrysoberyl cat's eye so expensive?
Fine chatoyant chrysoberyl is rare, and the combination of a sharp eye, milk-and-honey effect and good colour rarer still.
Does the eye have to be centred?
For quality, yes; an off-centre eye lowers the grade. It does not affect identity.
Is cat's eye durable?
Chrysoberyl at 8.5 is excellent for daily wear. Quartz at 7 is adequate; apatite at 5 is not.