The how and why of Tiger’s eye

Deborah Painter (USA)

Tiger’s eye is definitely an unusual semiprecious gem because of a phenomenon called “chatoyancy” seen in only a few minerals and stones. “Chat” is of course the French word for “cat”. The golden bands of polished specimens remind one of a cat’s eye. Chatoyancy refers to the way the distinctive bands of yellow and golden brown within the polished stone refract light as one tilts and moves the stone.

It seems to possess an inner dimension. That inner dimensionality effect is due to the fibres of crocidolite asbestos (a variety of magnesio-riebeckite) locked within the stone (Fig. 1). Magnesio-riebeckite is composed of silicon, iron and sodium. Quartz is composed of silicon and oxygen. Quartz has impregnated the greater portion of crocidolite within Tiger’s eye and only a small percentage is crocidolite asbestos. This is why Tiger’s eye is known as one of the “pseudomorphs”, which is a mineral that transforms partly into a different mineral.

A hydrothermal metamorphic process created the pseudomorph known as Tiger’s eye. In a hydrothermal condition during metamorphism of the bedrock in which crocidolite occurs, the mineral will experience tiny fractures. Quartz grows on the outer surface of the crocidolite. This process repeats itself until the crocidolite is surrounded completely in quartz and imbedded in it as the quartz grows within and adopts the fibrous structure; but the dimensions and supporting structure of the crocidolite remain the same. Blue Tiger’s eye has small quantities of un-oxidized crocidolite asbestos (Figs. 2 and 3).

The golden and red Tiger’s eye demonstrates crocidolite oxidation (Fig. 4). However, the sequence of stages of Tiger’s eye formation is still a subject of lively discussion among geologists.

Tiger’s eye is classified as a quartz chalcedony in the family of quartz compounds, such as jasper and onyx. Tiger’s eye comes in several colours, including reddish brown, golden brown, blue and grey. Some specimens might feature both reddish brown and grey. Tiger’s eye has a vitreous or silky lustre, a hardness of 7 on the Mohs scale of mineral hardness, and a specific gravity of 2.46 to 2.71. It is not fluorescent. It has a conchoidal (shell shaped) or splintery fracture (Fig. 5).

Tiger’s eye is commercially mined in Brazil, Myanmar, India, China, the United States (California), Australia and South Africa (Fig. 6). Once it was rare and expensive, until a large quantity was discovered in recent decades in Griqualand West, South Africa, the source for the specimen in Figs. 5 and 6. This discovery brought the price down and it is now an affordable stone for jewellery and small decorative items.

It is well known that asbestos is very hazardous to health. Being a variety of asbestos, does this mean that Tiger’s eye is hazardous to wear or touch?

No, Tiger’s eye is not at all hazardous to wear or handle. However, cutting, grinding or polishing it can pose problems when the finer particles are released to the air.

It is not just the fine fibres of crocidolite that make working with Tiger’s eye problematic without proper protection. Quartz is a hazardous mineral in its crystalline dust state. Silica dust is the culprit behind many fatal and near-fatal diseases, including pulmonary fibrosis, emphysema, lung scarring, and increased vulnerability to cancer and to tuberculosis. It usually takes years for serious symptoms like a chronic cough to develop in a person’s respiratory system and, by the time the symptoms have developed, the damage is irreversible.

In developed nations, workplace standards are being made stricter as governments and private organisations realise fully the danger of silica dust inhalation from sandblasting, breaking up concrete and yes, lapidary work. Many, many semiprecious stones worked with in the lapidary arts have quartz as their chief component. Opal, petrified wood, amethyst, carnelian and soapstone are just a few other mineraloids, minerals and fossils composed partially or mostly of quartz.

Trouble can easily be averted while doing lapidary work by wearing a United States National Institute for Occupational Safety and Health (NIOSH) approved respirator with dust filters and replaceable cartridges; if one is conducting lapidary work in the British Isles, the Health and Safety Executive office (HSE) has a list of approved respirators that have various designations. Some can block oil and some cannot. These respirators are designed to be comfortable on the face and washed with a mild soap and water. After about eight hours of use, change the cartridges.

The lapidary workshop should ideally be located outside but, if that is not feasible, a shed or garage equipped with a dust hood, suction fan and dust collector near the machinery will keep the finer dust from being breathed in.

A less expensive alternative to the above system is to use a wet/dry vacuum cleaner with a High Efficiency Particulate Absorbing or Arresting (HEPA) filter during the operation of the appliance. Place the vacuum cleaner outside the workshop or garage, because the vacuum cleaner has an exhaust on the opposite side from its hose and the exhaust will blow microscopic sized silica dust right back into the immediate environment.

Always use water or mineral oil as a lubricant for equipment to tamp down the “fugitive dust” when polishing, faceting or grinding. The wet/dry vacuum cleaner can be used with a bag in dry conditions and without one in wet conditions. The beauty of the vacuum cleaner is that it can clean the floor of the workshop afterward. Its interior barrel can be thoroughly washed with water and soap.

Be sure to wear clothes and a head covering; change clothes and put them in the laundry basket after finishing lapidary work. Washing exposed skin and hair is an extra measure to keep the silica dust and other particulates from being spread outside the workshop area.

About the author

Deborah Painter is an ecologist and general environmental scientist. She lives in the United States.

References

Gem Select website. February 7, 2022. Tiger’s Eye Gemstone Information: https://www.gemselect.com/gem-info/tigers-eye/tigers-eye-info.php.

Hamilton, Douglas. June 22, 2018. Dangerous Dust. Rock and Gem Magazine online: https://www.rockngem.com/dangerous-dust/.

HSE Health and Safety Executive Protective Equipment page https://www.hse.gov.uk/respiratory-protective-equipment/.

Oldershaw, Cally. 2003. Firefly Guide to Gems. Firefly Books. 229 pages.

Rice, Addison. 2023. International Gem Society. Gemstone Toxicity Table:

https://www.gemsociety.org/article/gemstone-toxicity-table/.

Vernon, Ron H. 2004. A Practical Guide to Rock Microstructure. Cambridge University Press. 594 pages.