Tiger Eye is a durable quartz composite. It begins as the fibrous blue mineral called crocidolite, which is comprised of iron & sodium. Most of us known crocidolite as asbestos. The transformation begins when quartz becomes imbedded between the fibers of crocidolite. This process will result in one of two gemstones. A blue stone called Blue Tiger Eye or Hawk Eye or the golden brown stone called Tiger Eye.
During the process, the asbestos is completely dissolved. But The quartz takes on the fibrous formations and the blue color of crocidolite. This creates the parallel lines within the gem which gives it that ever shifting play of light and movement, the stone is so loved for. This is also known as chatoyancy. This gleam that rolls across its surface. Much like the eyes of a cat.
Even though the iron & sodium dissolve, traces of hydrated oxide of iron deposit between the crocidolite and quartz, creating the golden color that is common to Tiger Eye. How much of this hydrated mineral is deposited will determine how Golden brown, red, green or blue, Tiger Eye and Hawk Eye will be. The rarer blue Hawk Eye will have only the slightest amounts.
The varying amounts of hydrated oxide of iron, actually cause several colors and mixes of color. When the color is a greenish gary, it is called cat-eye quartz. A golden yellow reflection on a brown stone, is called Tiger Eye. If the stone is blue gray or bluish, it's known as Hawk Eye. Redish brown, or mahogany colored stones, are known as bull-eye or ox-eye
Tiger Eye is also a pseudomorph (from the Greek for false form). Pseudomorphs form when one mineral replaces another. Tiger eye is a quartz replacement of crocidolite. Thus it is a pseudomorph of quartz after crocidolite. The process is similar to that which takes place when quartz replaces wood to produce petrified wood.