oxides
Tiger's eye
SiO2
- Also known as
- hawk's eyepietersitecrocidolite quartz
- Crystal system
- trigonal
- Hardness
- 6.5–7 on the Mohs scale
- Lustre
- silky
- Colour
- Golden brown with a moving silky band
- Where it forms
- By replacement of crocidolite, the asbestiform form of the amphibole riebeckite. Silica takes the place of the original fibres while preserving their alignment, which is why the light behaves as it does.
- Chiefly from
- Northern Cape, South AfricaWestern AustraliaIndia
Why it is that colour
Iron oxide staining. The optical effect — chatoyancy, the moving band of light — comes from a parallel fibrous structure inherited from the mineral the quartz replaced.
Safety
Finished polished stone is inert and safe to handle: the asbestos fibres have been replaced by silica and are no longer present as asbestos. Raw and partially replaced material is a different matter, and cutting or grinding either that or the parent crocidolite releases both asbestiform fibres and silica dust. This is a lapidary hazard, not a shelf one.
Where it comes from
Overwhelmingly from the Northern Cape of South Africa, from the same banded ironstone sequences that were also mined for crocidolite asbestos itself. That mining left a serious legacy of asbestosis and mesothelioma in the region, and the health story of the area and the stone are not separable. Blue and red material is often heat-treated or dyed.
Historical use
Not documented in ancient use. It enters the European market in the 1800s following the opening of the South African deposits, initially as a semi-precious curiosity.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Roman soldiers are frequently said to have carried it into battle. The claim circulates widely in crystal literature and is not supported by any classical source — the principal South African deposits were not worked until the nineteenth century. The protective and confidence associations are modern.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Sold as this, but not
Worth knowing
The pseudomorph is the point. A pseudomorph is a mineral that has taken over another mineral's shape — here, silica has replaced crocidolite fibre by fibre, keeping the alignment and losing the asbestos. The stone in your hand is a fossil of something genuinely dangerous, rendered inert by the substitution.
The Roman-soldiers story is a good example of how a plausible-sounding provenance becomes fact by repetition. It appears in crystal books, then in retail copy, then everywhere, and no version cites a source. The material comes from mines opened in South Africa in the 1800s. It is not that the Romans did not value amulets; it is that they could not have had this one.
Identity checked against IMA-CNMNC master list, June 2025.