Tiger's Eye
Actual Pieces found at
Serendipity Ranch Gem Mine
In
London in the mid-1870s, 25 shillings--about $85 in today's terms--went a long
way. You could buy 7 grams of gold, 40 liters of rum, or about a half kilogram
of opium. Where you couldn't get a bargain, however, was the jewelry store. That
same amount of money bought just 1 carat, or 0.2 gram, of a gem called
tiger's-eye. When rich sources of that precious stone were found in western
South Africa in the 1880s, prices plummeted. By 1900, tiger's-eye was considered
merely semiprecious. Today, a savvy shopper can purchase the gem for about $1.50
per carat.
The passage of time has transformed more than the gem's
price. Recent research has upended a 130-year-old theory about how tiger's-eye
forms. As a result, scientists soon will be scrambling to update everything from
mineralogy textbooks to museum displays.
Shining bright in its natural state, tiger's-eye is an
unremarkable rock with a dull sheen. When polished and illuminated, however, the
stone reflects a narrow band of light that changes position as the gem is turned
back and forth. This effect, called chatoyancy, gets its name from the French
phrase for "cat's eye" because of its resemblance to a feline's
slitted pupil. Chatoyancy occurs when light reflects from minute, parallel
ridges, fibers, or tubes within a transparent material.
Early in the 1800s, mineralogists recognized that tiger's-eye
was a fibrous variety of quartz, or silicon dioxide. In 1873, the German
mineralogist Ferdinand Wibel learned more. While studying the chemistry of
hawk's-eye, a blue form of tiger's-eye, he found that the gem was almost
entirely quartz but that it also contained fibers of crocidolite, an often
bluish, iron-bearing form of asbestos. Wibel proposed that hawk's-eye forms in
Earth's crust when quartz dissolved in hot water infiltrates spaces between
crocidolite fibers and then slowly replaces the asbestos' molecules. Brown
tiger's-eye, Wibel said, comes after yet another step. It results when chemical
reactions transform some of the iron in the bluish crocidolite into brownish
iron oxide.
The idea that tiger's-eye is a pseudomorph--a mineral in
which crystals of one material take on the form of another, which it replaces
atom by atom--held sway for more than 125 years. In fact, tiger's-eye is cited
in many textbooks as a classic example of a pseudomorph, says Peter J. Heaney, a
mineralogist at Pennsylvania State University in University Park. During his own
efforts to understand the processes underlying pseudomorphism, Heaney examined
thin samples of tiger's-eye under a microscope and realized that Wibel was
wrong.
Heaney expected to find that the quartz in tiger's-eye is
chalcedony, a form that typically consists of fibrous, defect-riddled crystals
less than 1 micrometer in diameter. Instead, Heaney was surprised to discover
relatively fault-free, column-shaped quartz crystals that measured more than 100
micrometers across and up to 10 millimeters in length. Pseudomorphism doesn't
produce such a uniform crystal form.
Heaney and his Penn State colleague Donald M. Fisher suggest
that the crystal structure of tiger's-eye forms via a so-called crackseal
mechanism. In such a process, quartz and crocidolite crystals simultaneously
condense from hot, mineral-rich fluids coursing through a tiny crack in a rock
and grow to fill it. Repeated episodes of fracturing lead to more cycles of
simultaneous, crack-filling growth of the two crystals.
In the tiger's-eye samples that Heaney studied, crocidolite
fibers often ran parallel to the quartz columns. In some cases, however, the
angle between the crocidolite and quartz was as much as 30 degrees. Because in
those instances, the reflected cat's-eye bands of light were perpendicular to
the crocidolite fibers, the scientists conclude that in tiger's-eye the
chatoyancy arises from the crocidolite fibers, not the quartz.