Gem library
Hawk's eye
The tiger's eye that stayed blue
Hawk's eye and tiger's eye are the same material at two stages of one transformation. The first is blue because the iron it contains has not yet rusted; the second is golden because it has.
- Hardness (Mohs)
- 6.5 – 7
- Density
- 2.64 – 2.71
- Crystal system
- Trigonal
- Refractive index
- 1.534 – 1.540
- Cleavage
- None
Mineralogical identity
- Family
- Fibrous quartz
- Species
- Quartz
- Variety
- Pseudomorph of crocidolite, blue stage
- Chemical formula
- SiO₂
- Crystal system
- Trigonal (fibrous aggregate)
- Classification
- Semi-precious stone
- Rarity
- Very common
- Indicative price
- €0.30 – 6/ct
Etymology
The name describes the reflection: a band of light that travels like a pupil, in a blue likened to a bird of prey's eye.
Physical & optical properties
- Hardness (Mohs)
- 6.5 – 7
- Density
- 2.64 – 2.71
- Refractive index
- 1.534 – 1.540
- Birefringence
- up to 0.009
- Cleavage
- None
- Fracture
- Splintery
- Lustre
- Silky
- Diaphaneity
- Opaque
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- blue, blue grey, blue green, steel grey
Formation, geology & history
One fibre replaced by another
It begins with crocidolite, a blue asbestos of the amphibole group, in fibrous veins within the banded iron formations of South Africa. Silica-rich waters pass through and replace it fibre by fibre: silica takes the amphibole's place while keeping its structure exactly. That is a pseudomorph, literally a false form, and it is what gives the material its silky look. As long as the iron from the crocidolite stays in the ferrous state, the stone keeps the blue of the original asbestos: that is hawk's eye. When it oxidises to ferric iron, the stone turns golden brown: that is tiger's eye. A single block often shows both, separated by a green transition zone.
Chatoyancy, and how to cut for it
Parallel fibres reflect light in a narrow band at right angles to their direction, the cat's-eye effect. To obtain it the lapidary must orient the cabochon so the fibres run parallel to its base: get the orientation wrong and the band vanishes or splits in two. The dome must be reasonably high, since too flat a surface dilutes the reflection. On decorative slabs the aim is the opposite, a cut that reveals the banding, those undulations that suggest crumpled cloth.
A safety question, answered plainly
The starting material is asbestos, and the question fairly comes up. The answer is that silicification replaces the amphibole fibres with silica: the finished product is no longer asbestos, and a polished stone releases nothing. Sawing or grinding a rough block that still holds untransformed crocidolite, on the other hand, produces a dust not to be breathed, which is true of all stone work and more so here. Workshops cut wet, and that is the correct practice.
How to recognise it
- The band of light movesTurn the cabochon under a single lamp: the band should glide at right angles to the fibres. A uniform blue with no band is not hawk's eye.
- Zigzag bandingOn a sawn face the fibres draw tight undulations, often folded. A smooth, structureless colour should raise doubt.
- Quartz hardnessIt scratches glass readily. A silky blue stone that steel can scratch is an imitation, probably dyed.
Imitations, treatments & confusions
- Tiger's eye dyed blueThe commonest fraud: golden material bleached and dyed. The dye gives too bright, almost electric a blue and pools in the hollows. True hawk's eye is greyish and muted.
- Fibre glass (cat's eye glass)Drawn glass sold as beads. The band is too crisp, too straight, and the stone is warm to the touch instead of cool.
- A mix sold as hawk's eye when it is pietersitePietersite is the same material but brecciated, hence swirled. Dearer, and different: not the same stone.
Deposits & origins
South Africa (Griqualand West, the reference deposit), Australia, Namibia, China, India, Brazil, Myanmar
Care & precautions
- Warm soapy water and a soft cloth, briefly rinsed and dried at once.
- No ultrasonics: fibre planes separate, especially in dyed stones.
- Light does nothing to it, but a dyed stone does fade. One more clue.
- Store separately, the silky polish scratches and loses its sheen.
Traditions & crystal healing
- Chakras
- Throat · third eye
- Zodiac signs
- Capricorn · Aquarius
- Element
- Air
It is credited with discernment and stepping back, as against tiger's eye, which is more associated with action. The two are often sold together for that reason. 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
Hawk's eye or blue tiger's eye?
Both names mean the same thing. «Blue tiger's eye» is often used for dyed material, so «hawk's eye» is the better term when the stone is natural.
Does it still contain asbestos?
A polished stone, no: silica has replaced the fibres. The precaution applies to dry working of rough material, which is never done without water in a workshop.
How do I check it is not dyed?
Look at the colour in hollows and cracks: dye accumulates there and forms darker lines. And be wary of too saturated a blue, the natural one is greyish.
Tools for this stone
Hawk's eye: only the tools this stone actually concerns, each carrying the value read on this page.