|Tigereye (tigerseye) is a replacement material kind of like a fossil. It is pseudomorph of silica which has partially or completely replaced a fibrous predecessor mineral (crocidolite).
Hawkseye is the pure blue variety of the material but is also called "blue tigerseye". Crocidolite is a variety of the mineral riebeckite and is in the asbestos family of fibrous minerals. The crocidolite has been partially or completely replaced with silica and the open spaces between fibers has been filled in with silica in hawkseye. It produces striking patterns of light when cut.
Tigerseye maybe a chemical change to the crocidolite where some of it is converted to limonite and other iron oxides. Tigereye is usually yellow-brown in color and displays a nice chatoyant pattern with light. It ranges from light yellow to medium yellow with brown in most samples.
Heating yellow-brown tigereye produces red tigereye. Since the red color is produced from the yellow starting material it is also possible to produce mixed colors with some tigereye having alternating areas of red and yellow patterns. This happens from zone heating.
As tigereye can comes from the chemical alteration of hawkseye it is also possible for some tigereye to show all three colors, yellow, red and blue, and the yellow and blue can combine to form a green variety. If the alteration products do not break down the fibrous nature of the stone, then all of it will display chatoyant patterns and it can be quite spectacular.
Binghamite is a similar material where chalcedony as encapsulated parallel fibbers of goethite and/or hematite. It has similar but more broken up appearance and often contain jasper. This material is found in Minnesota.
Pietersite is fundamentally brecciated tigereye. The fibers are broken up into clumps and then spread throughout the final stone. They too may have multiple colors and it produces a beautiful material. It is found in only two localities, Namibia and China.
The most sought after tigerseye comes from one locale in Australia and has been named "maramamba tigerseye". It often has veins of hematite/magnetite an jasper running through it. What really makes it stand out are the size of the fibers in the material. Some can reach several inches in length (4-5") whereas the more normal tigerseye tends to have fibers up to about 2"-3". It is found in only one locale in Australia, the Hamersley Ranges of the Pilbara region. For some superb images see the following site, run by Glenn Archer.