McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams

Home » Minerals » Chrysoberyl

Chrysoberyl


An extreme gem: The third-hardest gem mineral. The gem with the finest "cat's eye." The color-change mineral alexandrite


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.


The Diverse Gems of Chrysoberyl



Chrysoberyl is best known for its use as a gemstone. There are multiple varieties of 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.


Cat's Eye



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



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

Chemical Classification oxide
Color usually ranges from brown to pale yellow, yellow-green and green
Streak colorless
Luster vitreous
Diaphaneity transparent to translucent
Cleavage Poor, prismatic
Mohs Hardness 8.5
Specific Gravity 3.7 to 3.8
Diagnostic Properties hardness, color
Chemical Composition beryllium aluminum oxide, BeAl2O4
Crystal System orthorhombic
Uses 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.


Geologic Occurrence



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:



Find it on Geology.com




More from Geology.com


shale gas
Shale Gas is natural gas trapped within shale. It is a growing source of US supply.
Opal
Pictures of Opal: A collection of different types of opal from all around the world and Mars too!
Homeowners Insurance
Homeowners Insurance usually does not cover the most common geologic hazards.
US Diamond Mines
US Diamond Mines: Did you know that diamonds can be found in the United States?
Gems from Space
Gems from Space A number of materials from space have been used as attractive gems.
Novarupta
Wrong Volcano! The most powerful eruption of the 20th century was misidentified?
US Gemstones
United States Gemstones: A diversity of gemstones are produced in the United States.
Dallol Volcano
Strange Volcanic Landscape! Photos of Dallol Volcano - an explosion crater in Ethiopia.


Chrysoberyl
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.




Cat's Eye Chrysoberyl showing the milk and honey effect
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.


Color Change Alexandrite photo by David Weinberg for Alexandrite.net
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.


Chrysoberyl Crystal by Yaiba Sakaguchi
A beautiful chrysoberyl twinned crystal from Minas Gerais, Brazil. Photo by Yaiba Sakaguchi, used here under public domain.


More Minerals
  Mohs Hardness Scale
  Minerals
  Diamonds Do Not Form From Coal
  Fluorescent Minerals
  Minerals in Colored Glass
  Quartz
  Mineral Identification Chart
  The Acid Test


Chrysoberyl References
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.




© 2005-2014 Geology.com. All Rights Reserved.
Images, code and content of this website are property of Geology.com. Use without permission is prohibited. Pages on this site are protected by Copyscape.