Tiger’s Eye

How it formed

Tiger’s eye begins with quartz and an iron-rich fibrous mineral called crocidolite. In studied South African material, they grew together in openings within the rock. Water carried the dissolved ingredients into a fracture, where new quartz and fine amphibole fibres could grow.

As the opening filled, movement in the surrounding rock could split it again. More mineral-bearing water entered. The next growth sealed the new space, sometimes with the fibres slightly offset from the earlier ones. Repeated opening and filling built the banded texture.

The blue fibres could then change through weathering. Iron oxides and hydroxides developed as the amphibole altered, giving golden tiger’s eye its brown and yellow colours. Where the fibres remain less altered, the quartz can show the dark blue of hawk’s eye.

The moving sheen comes from the aligned mineral fibres. The quartz around them consists of many crystals. Under a small light, different areas brighten as the stone turns and brings their fibres into the reflecting angle.

This crack-and-seal interpretation was proposed for examined South African specimens. Older accounts describe quartz replacing crocidolite. The two accounts describe different growth histories; a surface photograph alone cannot establish which history a particular piece records.

How to look at yours

Turn a polished stone slowly beneath one small light. Follow the bright band across the surface, then move into diffuse daylight to see its body colour. Gold, brown and blue-grey areas may share the same piece.

Compare neighbouring bands. Their sheen can brighten at different angles because the internal fibres follow different directions. The shape and orientation of a cut also affect the reflection.

Treat colour and treatment as separate observations. A golden or blue appearance occurs in natural material, but the colour of an individual piece needs its own treatment record. The moving band of light identifies an optical effect; it does not certify origin or treatment.

One way to read it

One way to read this: a crack opens in the rock. Water enters, and quartz and fine mineral fibres grow together, gradually filling the gap. The rock moves again, opening another space for growth. Small shifts between successive growths leave bands within the stone. As you turn it, light catches the aligned fibres and moves across its surface.

This reading follows the crack-seal model proposed for studied South African specimens. Tiger’s-eye formation remains debated.

The name

The English name tiger-eye is in print at least as early as 1888, and a 1912 museum guide already separated tiger-eye from hawk’s-eye. The eye-like band of light seen across a suitably cut surface is what gemmologists call chatoyancy.

The dark blue appearance is called hawk’s eye or falcon’s eye. These are names for quartz appearances. Pietersite also shows a fibrous sheen, but its silica is chalcedony and its texture needs a separate geological account.

A field study near Prieska also records variegated tiger’s eye, where yellow and blue areas meet within the same material. Red colour can be produced by heating. These descriptions cover appearance and treatment, with no fixed worldwide count of varieties.

Tiger iron includes tiger’s-eye material within iron-rich banded rock. Its jasper and iron-oxide layers make it a different comparison from colour alone.