Tiger’s Eye is a popular semi-precious stone celebrated for its unique visual effect: a shimmering, shifting band of light across its surface. This distinctive optical phenomenon is known as chatoyancy, a term derived from the French word for “cat,” as the effect resembles a cat’s slit pupil. The rich, golden-brown colors and silky luster make it a favored material for jewelry and ornamental items. With a Mohs hardness of 7, its durability contributes to its frequent use in items like cabochons, beads, and ring stones.
Geological Identity and Composition
Scientifically, Tiger’s Eye is defined as a variety of chalcedony quartz, a microcrystalline form of the mineral silicon dioxide (SiO2). What sets it apart from common quartz is the presence of fine, parallel fibrous inclusions within the silica matrix. These fibers are the remnants of an amphibole mineral called crocidolite, which is a form of blue asbestos. The signature chatoyancy is a direct result of light reflecting off these perfectly aligned, altered mineral fibers trapped inside the quartz structure.
The fibers are generally subparallel to one another. When the stone is properly cut and polished, the light reflects uniformly to create the characteristic luminous band. The rich, golden-brown coloration is not inherent to the original fibers but comes from iron oxidation.
The Process of Formation (Pseudomorphism)
The existence of Tiger’s Eye is directly linked to a specific geological process of mineral replacement over millions of years. This process is traditionally described as pseudomorphism, where one mineral is chemically replaced by another while retaining the original mineral’s physical form. The precursor mineral is crocidolite, which is an iron-rich, blue fibrous amphibole mineral.
Groundwater rich in dissolved silica infiltrates the rock containing the crocidolite fibers. The silica then slowly dissolves the crocidolite and precipitates quartz in its place, preserving the delicate, parallel fibrous structure of the original mineral. This replacement results in a silica-based stone that is a “false form” of the original crocidolite.
The final, recognizable golden-brown color is achieved through the oxidation of the iron content released from the original crocidolite. This iron oxidizes to form limonite, a hydrated iron oxide, which coats the newly formed quartz fibers. The degree of this iron oxidation dictates the final hue, ranging from the deep, reddish-brown to the brighter golden-yellow tones.
Primary Global Deposits
The vast majority of the world’s commercially available Tiger’s Eye originates from a single, extensive deposit in South Africa. This primary source is located in the Northern Cape Province, specifically within the Griqualand West region. The deposits occur in ancient geological structures known as Banded Iron Formations (BIF), which date back to the Precambrian era, over two billion years ago.
The main mining area is centered around the town of Prieska, where the Tiger’s Eye occurs as seams or lenses interbedded within the host rock. South Africa’s arid climate and iron-rich terrain provided the perfect conditions for the extensive crocidolite formation and subsequent silicification and oxidation.
Mining operations in the Prieska area target these seams, which can vary in thickness and are often close to the surface. The scale of the South African production is so immense that it has historically dictated the global availability and pricing of the gemstone.
Secondary and Minor Occurrences
While South Africa dominates the global market, other deposits of Tiger’s Eye exist, though they are generally smaller in scale or less commercially viable. Australia is home to notable occurrences, particularly in the Western Australia region, where it is often found alongside other iron-rich minerals in a material known as Tiger Iron. This material is a decorative combination of Tiger’s Eye, red jasper, and black hematite.
Other countries contributing to the world supply include India and Namibia, both of which possess modest deposits. Minor occurrences have also been reported in locations such as China and the United States (Arizona and California).