Tiger's Eye

The photo shows three polished teardrop-shaped stones, with golden, brown and dark bands. Turn them, and the golden bands light up and go dark, as if the light were moving inside the stone. This effect is called chatoyancy, and according to Scientific American, the word comes from the French for "cat's eye". Tiger's eye itself, according to Minerals.net, is quartz, just like clear quartz, except that it is built of fine parallel fibers, which makes it opaque with a silky luster.

The stones in the photo also have bluish grey patches. According to Minerals.net and Hebrew Wikipedia, when the process that forms tiger's eye is incomplete, a bluish grey color remains, and the stone is called "hawk's eye". So each of these teardrops holds both a tiger and a hawk, which is quite common.

Now for the process itself. According to Scientific American, for more than a century textbooks explained that tiger's eye is a "pseudomorph": quartz that moved into crocidolite, a blue asbestos, and replaced its fibers one by one, a bit like bone turning into a fossil. The explanation seemed so obvious that nobody checked it closely. In 2003, Peter Heaney and Donald Fisher published a study in the journal Geology using an electron microscope, and proposed a different explanation: the quartz did not form fibers but tiny columns, and the shine comes from fine crocidolite fibers trapped inside them. In their view, a crack in the rock opened and sealed again and again, adding a layer each time. In 2004 the journal published a comment and a reply on the paper, so the debate is not entirely closed.

According to Scientific American, in the 1870s tiger's eye was worth about half an ounce of gold per carat. Then huge amounts of it were found in South Africa, and according to Minerals.net, most tiger's eye still comes from there, from the Northern Cape Province. The price dropped, and it has been one of the cheapest gemstones ever since. An investment that did not age well, but a stone anyone can hold.

According to Wikipedia, red tiger's eye is usually made by gentle heating, and dark stones are sometimes lightened with nitric acid. And according to Minerals.net, most tiger's eye on the market is natural, but some of the red stones are dyed. The common imitation, according to Wikipedia, is fiber optic glass, and it comes in a huge range of colors. If your tiger's eye is purple, it is worth asking some questions.