The popular stone with its deep brown to gold striations and shimmery glitter adorns jewelry throughout the world. Once considered a highly valued rarity in western Europe, the discovery of large sources in South Africa in the 1880s caused its value to plummet, but today, it is a popular, although inexpensive, stone.
Since 1873, geologists have believed that tiger's eye is an example of pseudomorphism. In pseudomorphism, the molecules of an original material are replaced by another material with the physical structure remaining unchanged. In petrified wood, for example, the material of the tree is replaced by minerals, but the tree shape and even the ring demarcations remain visible.
"I looked at tiger's eye because I expected to see pseudomorphism and wanted to understand better how the replacement took place," says Dr. Peter J. Heaney, associate professor of geosciences. "When I looked at a thin section, I realized that it was not pseudomorphism."
In the late 1800s, geologists thought that tiger's eye and the related blue-green stone, hawk's eye, were quartz replacements of crocidolite – a blue form of asbestos. They believed that quartz replaced most of the crocidolite, and the remaining asbestos provided the bluish color. They believed that in tiger's eye, the pseudomorphism went to completion and the rusting of iron in the asbestos caused the golden color.
"This theory had not been challenged since Wibel suggested it in 1873," Heaney says in a paper in the April issue of the journal Geology. "Surprisingly, tiger's eye also is one of the few gem materials that has virtually eluded modern micro analysis. Consequently, Wibel's interpretation has endured in standard mineralogy textbooks for 125 years."
Heaney and Dr. Donald M. Fisher, professor of geosciences, investigated tiger's eye, using light optical microscopes, x-ray diffraction techniques and transmission electron microscopes to determine the origin of the fibrous character that imparts chatoyancy to this gem. Chatoyancy is a band or star of reflected light or color that moves through a gem depending on its orientation. Whether in star sapphire or tiger's eye, a fibrous inclusion in the stone causes chatoyancy. Originally, tiger's eye's chatoyancy was attributed to chalcedony – a fibrous form of quartz – replacing fibrous crocidolite.
"Our examination of the quartz that constitutes tiger's eye has revealed without ambiguity that no chalcedony is present in the South African specimens we examined," says Heaney.
But, if tiger's eye is not pseudomorphic, another explanation must exist.
"The textures observed in tiger's eye do not offer convincing evidence for quartz pseudomorphism after crocidolite," says the Penn State researcher. "However, the fabrics are quite consistent with formation by a discontinuous crack-seal mechanism."
The crack-seal mechanism that forms hawk's eye and tiger's eye begins when a quartz rock containing some crocidolite cracks. Water containing the minerals necessary to make both quartz and crocidolite seep into the crack. Quartz begins to grow on the crack surface, while crocidolite fibers begin to grow from pieces of crocidolite that are on the crack surface. The crack area fills with new material and then the rock cracks again, usually in a sliding motion along the existing crack surface because it is weaker than other areas.
The new crack fills the same way, but the crocidolite fiber is slightly offset because of the crack shear. This process forms bands of quartz with diagonal, discontinuous crocidolite fibers creating hawk's eye. After some of the iron in the crocidolite oxidizes, the material becomes the familiar shades of brown and gold of tiger's eye.
Because crocidolite, and not quartz, is the cause of the shimmer in the tiger's eye, the variable chatoyancy that is tiger's eye's signature actually is a record of the structural path of the parent rock during tectonic events, says Heaney.