Tiger's eye is a quartz rock with a distinctive silky, wavy chatoyancy in bands of golden brown — the light seems to move across it as the stone is turned. It is inexpensive and abundant, chiefly from South Africa and Australia, and its imitations are correspondingly cheap: fibre-optic glass, which produces a chatoyancy of a different and, once seen, obvious kind.
What tiger's eye is
Quartz that has replaced, or grown around, fibrous crocidolite (a blue asbestos mineral), preserving the fibrous structure. In brown tiger's eye the fibres have oxidised to golden iron oxide; in blue "hawk's eye" the original blue crocidolite remains; "pietersite" is a brecciated, swirled variant. Properties: RI 1.544–1.553 (quartz; aggregate spot reading), SG 2.64–2.71, hardness 6.5–7, chatoyant in undulating bands, inert under UV.
The asbestos is fully enclosed in quartz in finished, polished pieces and poses no hazard in normal handling; the concern applies to cutting and grinding.
Its varieties
- Golden / brown tiger's eye — the standard; golden-yellow to brown bands.
- Hawk's eye — blue-grey, the unoxidised form.
- Red tiger's eye — brown tiger's eye that has been heated, turning the iron oxide red. A treatment, usually undisclosed; essentially all red tiger's eye is heated.
- Pietersite — brecciated tiger's eye with swirling chatoyancy; natural, from Namibia and China.
- Tiger iron — tiger's eye banded with hematite and jasper; natural.
Imitations
Fibre-optic glass. Bundles of parallel glass fibres fused into a block and cut, producing a sharp, straight, bright chatoyant band — a single "eye" rather than the undulating, multi-directional silk of tiger's eye — in any colour, including colours tiger's eye never shows (blue, pink, green, red). Under magnification the fibres are perfectly regular and hexagonally packed; the material is singly refractive, SG about 2.5, and bubbles may be present. Extremely common in cheap beads and cabochons.
Dyed tiger's eye. Blue, green and other colours dyed onto pale tiger's eye; dye in fractures; unnatural colour.
Plastic. Printed or moulded stripe pattern with no movement of light.
How the laboratory identifies it
The character of the chatoyancy is the first test: tiger's eye's silk is wavy, banded and shifts in complex ways; fibre-optic glass shows a single straight sharp line. Magnification confirms — natural fibrous structure with irregularity versus mechanically regular fibres. SG (2.68 versus 2.5) and the polariscope (aggregate reaction versus glass strain) support. Colour outside the natural range indicates dye or glass. Heating (red) is inferred from colour and reported as a treatment.
Frequently asked questions
Is tiger's eye safe to wear?
Yes. The asbestos fibres are locked in quartz in polished pieces. Do not grind or sand it without protection.
Is red tiger's eye natural?
The stone is natural; the red is produced by heating. It should be sold as heat-treated.
How can I spot fibre-optic glass?
A single dead-straight, razor-sharp bright line, especially in a colour like blue, pink or green, is glass. Tiger's eye's light moves in soft, wavy bands.
Is blue tiger's eye dyed?
Natural blue hawk's eye exists and is greyish blue with the same silky structure. Bright, saturated blue is dyed or glass.
Does tiger's eye need treatment disclosure?
Only the red (heated) variety and dyed colours. Golden tiger's eye is sold untreated.