This minor ore of beryllium is one of the most important gem minerals.
What is Beryl?
Beryl is a relatively rare silicate mineral with a chemical composition of Be3Al2Si6O18. It is found in igneous and metamorphic rocks in many parts of the world. Before 1969, beryl served as the only important ore of beryllium metal. Since then, most of the world's supply of beryllium is refined from bertrandite, a beryllium silicate hydroxide, mined at Spor Mountain, Utah. Small amounts of beryl, mostly produced as a by-product of gemstone mining, are still used to produce beryllium.
The major economic interest in beryl today is its use as a gemstone. It is one of the most important gem minerals, and the gems are named by their color as emerald (green), aquamarine (greenish blue to blue), morganite (pink to orange), red beryl (red), heliodor (yellow to greenish yellow), maxixe (deep blue), goshenite (colorless), and green beryl (light green). Emerald and aquamarine are the most popular. Compared to other gemstones, emeralds are second only to diamonds in terms of the dollar value imported into the United States. Occasionally, chatoyant specimens of beryl are found that can be cut into cabochons to produce interesting cat's-eye gemstones.
Geologic Occurrence of Beryl
Beryl is a mineral that contains a significant amount of beryllium. Beryllium is a very rare metal, and that limits the occurrence of beryl to a few geological situations where beryllium is present in sufficient amounts to form minerals. It mainly occurs in granite, rhyolite, and granite pegmatites; in metamorphic rocks associated with pegmatites; and, in veins and cavities where hydrothermal activity is associated with rocks of granitic composition. These different types of deposits are often found together and serve as an exploration model for finding beryl.
Beryl is also found where carbonaceous shale, limestone, and marble have been acted upon by regional metamorphism. The famous emerald deposits of Colombia and Zambia have been formed under these conditions. The carbonaceous material is thought to provide the chromium or vanadium needed to color the emerald.
Physical Properties of Beryl
|Color||Green, yellow, blue, red, pink, orange, colorless|
|Streak||Colorless (harder than the streak plate)|
|Diaphaneity||Translucent to transparent|
|Mohs Hardness||7.5 to 8|
|Specific Gravity||2.6 to 2.8|
|Diagnostic Properties||Crystals are prismatic with flat terminations, hexagonal, and without striations. Hardness and relatively low specific gravity.|
|Crystal System||Hexagonal (occurs in prismatic to tabular crystals)|
|Uses||Gemstones, a minor ore of beryllium.|
Physical Properties of Beryl
The most important physical properties of beryl are those that determine its usefulness as a gem. Color and clarity are very important. Beryl occurs in a diversity of colors, with some of those colors being highly desirable. It also occurs in transparent crystals that have clarity and size that are sufficient for faceting. Many beryls have a color that can be improved by heating.
Beryl's durability is generally good. It has a Mohs hardness of 7.5 to 8, which helps it resist scratches when worn in jewelry. Beryl breaks by cleavage and it is also brittle. Many specimens, especially of emerald, are fractured or highly included. These weaknesses can make it vulnerable to damage by impact, pressure or temperature change.
Beryl can be difficult to identify. When it occurs as a crystal, its prismatic, hexagonal form with flat terminations and lack of striations is a good aid in identification. Beryl's high hardness and relatively low specific gravity can be helpful for identifying massive specimens.
Uses of Beryl (ore of beryllium)
Beryl was once the only important ore of beryllium metal, but today the mining of bertrandite at Spor Mountain, Utah supplies about 80% of the world's beryllium. The extraction of beryllium from beryl is very costly, and as long as bertrandite is available in large amounts, beryl will be a minor ore of that metal.
The primary economic use of beryl today is as a gemstone. It occurs in a wide variety of colors that appeal to many consumers. A brief description of popular gem beryl varieties is presented in the sections below.
Emeralds are gem-quality specimens of beryl that are defined by their green color. To be considered an "emerald," a stone must have a rich, distinct color in the bluish green to green to yellowish green range. If the color is not a rich saturated green, the stone should be called a "green beryl" instead of an "emerald."
There are some disagreements between buyers and sellers on judging the color boundary between emerald and green beryl. Some also believe that the name "emerald" is reserved for stones with a green color caused by chromium rather than by vanadium. Material colored by iron is almost always too light to be called emerald and usually lacks the distinct green color typically associated with emerald.
Emerald is the most popular beryl. Excellent specimens are also quite valuable. Emerald, sapphire and ruby are considered to be the "big three" of colored stones. More money is spent on these in the United States than all other colored stones combined. In many years, the United States imports a higher dollar value of emerald than of ruby and sapphire combined. Colombia, Zambia, Brazil, and Zimbabwe are major producers of gem-quality emerald. A small amount of emerald is sporadically mined in the United States near Hiddenite, North Carolina.
Emerald is a beautiful stone, but it is often fractured or highly included. Most of the emerald entering the retail market has been treated in some way. Fractures are often impregnated with glass or resins to stabilize the stone and make the fractures less visible. Stones are often waxed or oiled to hide fractures and surface-reaching inclusions. Heating and drilling are often done to reduce the visibility of inclusions.
Even after these treatments, a person with a small amount of knowledge can usually look into a display case at the typical mall jewelry store and with reasonable success identify natural stones and lab-created stones by their clarity. Lab-created stones have a bright green color and are transparent. Natural stones are usually translucent or have visible inclusions and fractures. Natural stones without these characteristics are extremely rare and have a very high price. Many people prefer natural stones and their visible flaws. Others prefer the clarity and color of lab-created stones and their significantly lower price. Lab-created emeralds account for a significant percentage of the stones on display and being sold in many department stores and mall jewelry stores.
Aquamarine is the second most popular gem beryl. Like emerald, its identity is defined by its color. Aquamarine has a distinct greenish blue to blue color. Unlike emerald, light-colored stones in this color range are still called aquamarine. The stones that are richly colored are the most desirable, and the stones with a very pale color are made into inexpensive jewelry. Aquamarine differs from emerald in another way - it normally has far fewer inclusions and fractures. So, most of the aquamarine seen in mall jewelry stores is usually eye clean and without visible fractures.
The color of aquamarine can usually be improved by heating. Most stones entering the retail market have been heated. Many of the greenish blue stones offered for sale were distinctly bluish green or even yellow before treatment.
Morganite, also known as "pink beryl" and "rose beryl," is a rare variety of beryl that ranges in color between yellowish orange, orange, pink, and lilac. "Rose," "salmon," and "peach" are words often used to describe the gem's color. Trace amounts of manganese are thought to cause the pink color.
Morganite is the third most commonly seen variety of beryl in jewelry stores, but the selection is usually limited, and stones with top color are very hard to find. Most morganite sold in jewelry has been heat treated to improve its color. Heating generally removes traces of yellow from the stone and converts orange or yellowish stones into a more desirable pink color. Some morganite has been irradiated to deepen its color. Synthetic morganite has been produced but has not been widely marketed because morganite is not well known to consumers.
Three things have limited the popularity of morganite: 1) most specimens have a very light color; 2) jewelry manufacturers are hesitant to make a large commitment to the gem because they usually do not have a steady source of supply; and, 3) consumers are not familiar with morganite because it has never been strongly promoted.
Yellow beryl, also called "golden beryl" or "heliodor," is a yellow to greenish yellow beryl. Yellow beryl is a durable stone that often has a beautiful yellow color and a relatively low price. The public is not familiar with the gem, and as a result the demand is very low and so is the price. People who enjoy yellow gems and want an item of jewelry with a yellow beryl will have a hard time finding it at most jewelry stores. It is most often seen in the inventory of a jeweler who does custom designs.
A few vendors call it "yellow emerald," but many believe that name is inappropriate because it is an incorrect use of the varietal name “emerald.” In fact, the Federal Trade Commission has proposed revising its Guides for the Jewelry, Precious Metals and Pewter Industries to state that incorrect usage of varietal names is “unfair” and “deceptive.” Their proposal points directly to “yellow emerald” as an example of a misleading name. [To view the exact words of the Federal Trade Commission on this matter, see Section V. Varietals on page 7 of this document.]
Small amounts of iron are thought to produce the color of yellow beryl, which can often be changed with heating or irradiation. Despite the fact that many specimens of yellow beryl depreciate with treatment to less valuable colors, some specimens can be heated to a greenish blue similar to aquamarine, while others can be irradiated to produce a more desirable yellow color. Those with plans to treat yellow beryl must experiment because treatment success is variable.
"Green beryl" is the name given to light green specimens of beryl that do not have a tone and saturation dark enough to merit the name "emerald." Some of this light green beryl is colored by iron and lacks the distinct green color associated with emerald. Some is colored by chromium or vanadium and does not have the proper hue, tone, and saturation to be called "emerald."
The price difference between green beryl and emerald is significant, so some buyers or sellers hope to have specimens judged in their favor. This can lead to problems because a precise color boundary between emerald and green beryl has not been defined with industry-wide agreement. Green beryl can be an attractive gem, but it is rarely seen in jewelry.
Red beryl is one of the world's rarest gem materials. Gem-quality material that is large enough to facet has been found in very modest amounts in the Wah Wah Mountains and Thomas Range of Utah. Occurrences of red beryl have been found in the Black Range of New Mexico, but crystals there are just a few millimeters in length and are generally too small to facet.
Red beryl is usually a very strong and attractive red color. It has a high enough saturation that even small gems have a very strong color. This is fortunate because most gems cut from red beryl are very small and only suitable for cutting into melee. Gems over one carat in size are very rare and sell for thousands of dollars per carat. The material is often included and fractured, and these characteristics are accepted just as they are accepted in emerald.
In Utah, the host rocks of red beryl are rhyolitic lava flows. Here, crystals of red beryl formed in small vugs and shrinkage cracks long after the rhyolite crystallized. It is thought that ascending beryllium-rich gases encountered descending mineral-rich groundwater to create the geochemical environment needed to form red beryl. Trace amounts of manganese are thought to cause the color.
Beryl is a relatively rare mineral because beryllium rarely occurs in large enough quantities to produce minerals. Red beryl is extremely rare because the conditions needed to supply the color-producing manganese at the proper time to a beryl-forming environment is improbable. So, the formation of red beryl requires the coincidence of two nearly impossible events.
Red beryl was initially named "bixbite" after Maynard Bixby, who first discovered the material. That name has been mostly abandoned because it was so often confused with bixbyite, a manganese iron oxide mineral also named after Mr. Bixby. Some people call it "red emerald," but that name is rejected by many in the trade because it causes confusion with another variety of beryl named "emerald."
Goshenite is the name used for colorless beryl. In most cases, color in beryl is caused by trace amounts of certain metals that impart a color. That is often the case for goshenite, but color-inhibiting elements can keep goshenite colorless.
Goshenite is often found in large hexagonal crystals with exceptional clarity and transparency. In the Middle Ages these crystals were cut and polished into lenses for hand magnifiers, telescopes, and some of the earliest eyeglasses. With a Mohs hardness of 7.5 to 8.0, these were some of the earliest scratch-resistant lenses.
Goshenite is sometimes cut into gemstones. These gems are mainly of interest to collectors. They are rarely used in jewelry, because they lack color and their appearance is inferior to other colorless gems such as diamond and white sapphire.
Another rare beryl is a very dark blue material known as "maxixe" (pronounced mashish). The dark blue color is thought to be developed in the ground by exposure to natural radiation. Maxixe is an unfortunate material because the wonderful blue color quickly fades in daylight to a pale brownish yellow color. The color can be restored with additional irradiation, but that color is also quickly lost with exposure to light. Maxixe was first found in 1917 in a mine in the Minas Gerais area of Brazil. It has since been found in small amounts at a few other locations.
Beryl occasionally contains a fine silk that allows it to be cut into a chatoyant gem. Aquamarine, golden beryl, and emerald are the most likely beryls to be found with chatoyance. When properly oriented and cut en cabochon, these gems usually produce a weak cat's eye, but occasionally a strong cat's eye is produced.
The most desirable chatoyant beryls are those with a highly desirable color and a bright, thin eye that perfectly bisects the gem.
Author: Hobart M. King, Ph.D.
|Rock, Mineral and Fossil Collections.|
More From Geology.com: