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Tiger's Eye (Again)

Tiger's Eye is the common name for a variety of Chalcedony. This gemstone contains an intergrowth of amphibole fibers (also known as asbestos, the term for any of several minerals that will readily separate into long flexible fibers). In Tiger's Eye, crocidolite is the asbestiform of the mineral riebekite.

The fibers create the characteristic chatoyance of this gem-stone. When turned towards the light, it displays a series of lustrous bands alternating with bands of duller color. Throughout its metamorphic history with folding and faulting, the display is enhanced by the fact that the fibers are not always perfectly straight. They may be curved and often have a sharp bend or twist.

While Tiger Eye has been considered to be pseudomorph, where the crocidolite fibers were replaced with quartz ,much like the replacement that happens in petrified wood, studies at Penn State U. suggest otherwise. It is thought that this new evidence proves this may not be the case and that quartz and the crocidolite coexist, through a process called crack seal vein filling, where crocidolite grew within the fracture in the rock, and then were engulfed and encapsulated by a quartz enriched solution. The process repeated over and over through new fractures, resulting in the layered bands of undulating fibrous intergrowths.

Tiger's Eye is a 6.5 to 7 on the Mohs hardness scale. Notable locations for the gem stone are Australia, Africa, Virginia and Minnesota. As a matter of interest, it is noted that Tiger's eye was also used in cigarette filters. It was taken off the market when several deaths occurred (due to the asbestos content).

Cutting and Polishing tips: Find the flash! Orientation of cutting is critical to getting good chatoyance and color out of tiger eye. Cuts must be exactly parallel to the length of the fibers to get full chatoyance. If the saw cut is perpendicular, or 90% to the fibers, you end up with a lifeless, dark brown to black stone with no light play at all. And because of the asbestos, always wear a mask when cutting this material. The material can be treated with bleach. Colors can vary from tan—green—blue or brown.

The blue variety of Tiger's Eye has also been known as Hawk's Eye or falcon's Eye. Bull's Eye or Ox Eye has a relatively dark mahogany red as its chief color. Mara Mamba is considered the “Rolls Royce “ of tiger eye collections. The location is the Hamersley Ranges of the Pilbara region in Australia (found in only two deposits and mined out many years ago.

Pietersite seems to be differentiated by more swirls, swaths and other fibrous segments, rather than the continuous bands or fibers. Named for the prospector, Sid Pieters for his discovery in 1962 in northern Namibia. Chinese Pietersite, or Eagle's Eye, is named for the variety discovered in 1993 at Nangang, Hunan Province, China. It differs in coloration through itsd magnesium-rich amphibole.

Binghamite (or Cuyunite) - described as "an iron heavy rock that has inclusions of the familiar tiger eye fibers" but it is basically fibrous quartz in which the replaced fibers aare a red hematite and yellow goethite iron rather than crocidolite. Named after Bill Bingham, a lapidarist from St. Paul, who discovered it in 1936. The Secondary name refers to its sole locality in the Cuyuna iron range near Crosby, Minnesota. Silkstone refers to a variety of binghamite, where the pattern of the stone is arranged in level lines as if "enough tiger eye is present to dominate the stone".

Tiger iron - a banded ironstone rock composed chiefly of Tiger's Eye, red jasper, and black hematite, primarily found in the Ord Ranges near Port Hedland in Western Australia's Pilabara region. If it is mostly jasper, it is called Tiger's Eye Jaspelite.

From many sources—The Council Reporter Oct 2012, CMS eTumbler. 8/12, West Seattle Petroglyphs 6-=7/12 and from Maplewood News.