What Is Chrysoberyl?
Chrysoberyl is a beryllium-aluminum oxide mineral with a chemical composition of BeAl2O4.
It is distinctly different from the beryllium-aluminum silicate (Be3Al2(SiO3)6
mineral known as "beryl," although the similar names can cause confusion.
Chrysoberyl is not found in deposits that are large enough to allow it to be used as an ore of beryllium. Its only
important use is as a gemstone; however, it excells in that use because of its very high hardness and its special
properties of chatoyance and color change.
The Diverse Gems of Chrysoberyl
Chrysoberyl is best known for its use as a gem. There are multiple varieties of gem chrysoberyl, each with its
own name and unique physical properties.
Ordinary chrysoberyl is a yellow to yellow-green to green gemstone with a translucent to transparent diaphaneity.
Transparent specimens are usually cut into faceted stones. Specimens that are translucent or with silk are usually
cut into cabochons. A photo of ordinary chrysoberyl is shown at upper right.
Chrysoberyl is the gemstone that produces the most distinct "cat's eye," or chatoyance. If a person uses the name "cat's
eye" without the name of another gemstone (for example, "cat's eye tourmaline"), then he is most likely referring to
chatoyant chrysoberyl. Cat's eye chrysoberyl has also been called "cymophane."
The phenomenon of cat's eye occurs in cabochon-cut stones that contain a high density of parallel fibrous inclusions.
The "cat's eye" is a line of light that reflects from the dome of the cabochon at right angles to the parallel inclusions.
The line of light is very similar to how a spool of silk thread will produce a line of reflection across the top of the spool as it is moved back
and forth under a source of light.
Some specimens of cat's eye will appear to have a different color on each side of the cat's eye line when illuminated
from the proper direction with respect to the observer's eye. It gives the illusion that the stone is made of two different
materials, a light material on one side of the line and a dark material on the other. This phenomenon is known as the
"milk-and-honey" effect. A photo of cat's eye chrysoberyl showing the milk-and-honey effect is shown in the right column.
Alexandrite is the color-change variety of chrysoberyl. The most distinctive specimens appear to have a green to blue-green color
in daylight but change to a red to purplish-red color under incandescent light. Specimens with strong and distinct
color-change properties are rare, highly desirable, and sell at very high prices.
Stones over five carats are especially rare. A photo pair of an alexandrite gem in daylight and incandescent light is shown in the right column of this page.
The change in color is thought to occur only in specimens where chromium substitutes for aluminum in the mineral's atomic
structure. The chrysoberyl in which this phenomenon was first observed was named "alexandrite" after Tsar Alexander II of Russia. Since then the "alexandrite effect" has been observed in other gems, which include color-change garnet, spinel, tourmaline, sapphire and fluorite.
Alexandrite is a rare material, only found in very small deposits. It was first discovered in the Ural Mountains of Russia
in the late 1800s. Although that deposit has been mined out, small deposits have since been discovered in Brazil, India,
Sri Lanka, Myanmar, China, Zimbabwe, Tanzania, Madagascar, Tasmania and the United States.
Alexandrite can also be strongly pleochroic (a stone that has a different apparent hue when viewed from different directions).
It is a trichroic stone (exhibiting three different hues from three different directions) with a green, red or yellow-orange
hue depending upon the direction of observation. The pleochroism of chrysoberyl is not apparent in all specimens and varies
under different types of light. It is not as distinctive as the color-change effect.
Physical Properties of Chrysoberyl
||usually ranges from brown to pale yellow, yellow-green and green
||transparent to translucent
||3.7 to 3.8
||beryllium aluminum oxide, BeAl2O4
||as a gemstone: chrysoberyl when transparent, "cat's eye" when chatoyant, and "alexandrite" in specimens that exhibit color change
Physical Properties of Chrysoberyl
One of the most distinctive properties of chrysoberyl is its exceptional hardness. With a Mohs hardness of 8.5, it is the
third-hardest gemstone and the third-hardest mineral that is even occasionally found at Earth's surface. Although chrysoberyl
is extremely hard, it does break with distinct cleavage in one direction and indistinctly or poorly in two others. It also
has a brittle tenacity.
Most specimens of chrysoberyl are nearly colorless or fall into the brown to yellow to green color range. Red specimens
are occasionally found.
Chrysoberyl often occurs in tabular or prismatic crystals with distinct striations (see photo in the right column of this page). It also occurs in twinned crystals with
distinct star and rosette shapes. These crystals usually persist well and retain their shape during stream transport because
of the mineral's exceptional hardness. This makes them easy to identify in gem gravels, but the twinning often interferes with
their usefulness as gems.
As a beryllium mineral, chrysoberyl only forms under those conditions where large amounts of beryllium are present. This limits
its abundance and geographic distribution. High concentrations of mobile beryllium most often occur on the margins of magma bodies
during the final stages of their crystallization. Thus, chrysoberyl usually forms in pegmatites and in metamorphic rocks associated with pegmatites. These include mica schists and dolomitic marbles.
Chrysoberyl is also found along with other gem minerals in placer deposits. It is a hard, weathering-resistant mineral with a
high specific gravity. These properties allow it to survive in sediments after other minerals have been destroyed by abrasion
and chemical weathering.
Contributor: Hobart King
Find it on Geology.com
More from Geology.com
|Troglobites are creatures that have adapted to a permanent life in the darkness of a cave.
|Tanzanite was unknown until a few decades ago but it has erupted into wide popularity.
|Three faceted chrysoberyls showing a rage of yellow and yellow-green color. These stones were produced in Sri Lanka and are about 4.3 millimeters in diameter and weigh about 0.52 carats each - a very high weight for stones of this size caused by chrysoberyl's high specific gravity.
|Chrysoberyl that contains large numbers of fibrous inclusions can produce a "cat's eye," a line of light across the surface of the stone that orients perpendicular to the included fibers. Chrysoberyl is the gem that exhibits the finest cat's eyes, and when the term "cat's eye" is used without a mineral name as a modifier, the speaker is most likely referring to chrysoberyl. This specimen exhibits the "milk and honey" effect - when properly oriented, the stone has two apparently different colors on each side of the cat's eye line. This green cat's eye chrysoberyl was produced in Sri Lanka and is about 5.6 x 4 millimeters in size.
|A faceted specimen of color-change alexandrite of 26.75 carats from Tanzania showing a blue-green color in daylight and a purple-red color under incandescent light. Photographed by David Weinberg for Alexandrite.net and published here under a GNU Free Document License.
|A beautiful chrysoberyl twinned crystal from Minas Gerais, Brazil. Photo by Yaiba Sakaguchi, used here under public domain.
1) Chrysoberyl: Daniel Marshall and Lori Walton, IN: Mineralogical Association of Canada, Geology of Gem Deposits, Second Edition, Lee Groat, editor, pages 305-374, 2014.
2) Chrysobery and Alexandrite from the Pegmatite Districts of Minas Gerais, Brazil: Keith Proctor; Gems & Gemology, pages 16-32, Spring 1988.
3) Chrysoberyl: Michael O'Donoghue, editor, Gems, Sixth Edition, Chapter 7, pages 167-171, 2006.