How Is Tiger's Eye Formed?

Tiger's eye or Tiger Eye is a gorgeous, chatoyant, semi-precious gemstone with alternating banks of golden to red-brown colors and a silky luster. It has a fascinating and constantly shifting light effect in which the gold and brown bands appear to keep reversing as the stone is moved relative to the light source. This effect is caused by the light reflecting off of microscopic fibers of crocidolite (a blue form of asbestos) within the stone.

A polished stone sphere of Tiger's Eye.
A polished stone sphere of Tiger's Eye.


Since the 1870’s, geologists believed that tiger's eye was a pseudomorph where quartz replaced crocidolite - a blue form of asbestos. They believed that quartz replaced most of the crocidolite, and the remaining asbestos provided the bluish color. The stone then weathered/rusted causing the surfaces of the remaining crocidolite to become coated in iron oxides creating the golden color.

This theory was accepted as fact until the early 2000’s when some scientists studying pseudomorphs put a piece of South African tiger’s eye under a scanning electron microscope. They found no evidence that tiger’s eye was a pseudomorph, but rather proposed it was formed through a discontinuous crack-seal mechanism.

The crack-seal mechanism that forms 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.


References:
https://www.eurekalert.org/pub_releases/2003-04/ps-sot032803.php
https://blogs.scientificamerican.com/rosetta-stones/tiger-s-eye-a-deceptive-delight/?redirect=1
http://www.quartzpage.de/eyes.html

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