The last article on the gemstones of the beryl group closes with morganite as the subject. I think it appropriate now to review the nature and characteristics of the beryl group.
Beryl is a mineral of a class of minerals called silicates. Silicates are minerals composed primarily of silicon and oxygen (SiO4) tetrahedra. The tetrahedra can remain independent (orthosilicates or nesosilicates) or bond in various ways to form sorosilicates (two linked groups), inosilicates (single or double chains), phyllosilicates (flat sheets), cyclosilicates (rings) or the framework silicates known as tectosilicates. Mineralogist Paul Hlava describes the ring structure of beryl as “an array of tires stacked one atop the other, with sheets of beryllium and aluminum ions between the layers.” The ring structure of beryl produces six-sided prisms of moderate density and above average hardness that frequently exhibit numerous striations parallel to the length of the crystal.
Beryl is usually found in pegmatitic regions of granite and silica-poor mica schists or in the alluvial debris of such eroded areas. The hardness of beryl, its chemical resistance, and the lack of planes of definite cleavage aid in the preservation of the crystals in alluvial environments. Rarely does beryl occur in high temperature cavities (druses), where it precipitates from very hot beryllium-bearing solutions or gases. Rod-like masses are also uncommon. The crystals usually develop as elongated hexagonal prisms.
However, the alkali-rich morganite is the exception, as it frequently occurs as short stubby crystals and in tabular form. Dr. Frederick H. Pough has stated that “tabular beryl crystals have been found in abundance in New Mexico at Dixon.” The Pala and Mesa Grande districts of San Diego County in California produce crystals of superb quality. Deposits in Maine yield material associated with the tourmaline and other gem varieties found there. The pegmatites of the island of Madagascar (Malagasy Republic), particularly the deposits at Anjanabonoina, are the source of some very fine crystals.
In the Eyewitness Handbook series, Cally Hall in Gemstones lists the recently discovered deposits in Pakistan, as well as the productive areas in Mozambique, Zimbabwe, Namibia, and the island of Elba (Italy). Illustration #18 in Crescent’s Color Treasury of Crystals shows a picture of a very well developed 9mm. morganite crystal from Elba, now on display in the Natural History Museum of Milan.
Pure beryl is colorless goshenite. However, the channels formed by the stacked rings, where they are parallel to the C axis, can accommodate various ions, atoms, and molecules. Aquamarine, emerald, heliodor, green beryl, and morganite crystals develop when the structure contains the appropriate traces of other elements or minerals. Cally Hall mentions bi-colored Brazilian crystals composed of aquamarine and morganite.
Vorobyevite, vorobievite, worobieffite, rosterite, and rose beryl are terms used in earlier publications to designate pink beryl, now commonly referred to as morganite. The noted Tiffany gemologist, Dr. George F. Kunz, named the little known gem after the affluent New York banker and gem collector, John Pierpont Morgan. Later, the extensive Morgan gem collection was donated to, and now displayed at, the American Museum of Natural History in New York.
Various authorities have named cesium, lithium, and manganese as the color-producing agent in morganite. Dr. J. Kourimsky in The Illustrated Encyclopedia of Minerals and Rocks attributes the color to cesium, and Dr. Pough refers to “pink cesium beryls.” Dr. Kurt Nassau states in his Gems Made by Man, that the presence of manganese in natural beryl produces morganite. The Illustrated Dictionary of Jewelry by Harold Newman defines morganite as “rose-pink, owing to the presence of lithium.” In Gems in the Smithsonian, Paul Desautels also claims that lithium “accounts for the color of pink beryl.” Extensive research within the last few years has brought such dogmatic statements into question.
The following is a direct quote from a consultation with mineralogist Paul Hlava, a leading authority on crystal structures, their composition, physical and optical properties, and the interaction of light with the structures and their properties. He emphasized that “this is not a thorough and rigorous discussion of the causes of color in minerals or gems, which is a very complicated subject in itself.”
“It is now known that the production of color by trace elements often involves the excitation of electrons of the transition metal elements by absorption of specific energies from light”, states Paul Hlava. “The electrons move from their normal, ground state to a higher energy level in the atom. The color we see is the “left over” light that has not been absorbed. The electrons later fall back to the ground state by releasing this energy in several steps and do not cancel the absorption process. The density of the packing of the atoms in a crystal structure affects the distance between the electron energy levels and influences what color we perceive. Manganese is a `transition’ element and, though other impurities may be present, it is likely that it is responsible for our perception of the different shades of morganite. The color may also be modified by the presence of other transition metal element impurities, such as iron. It would appear that no mineral/gem is actually colored by lithium or cesium, although these elements are often found in colorful minerals. The lithium and cesium have no more effect on the color than the silicon and aluminum that are also present in these colorful minerals.”
Many gemstone reference books provide numerous descriptions of morganite’s colors. The International Student Edition of Mineralogy by Kraus, Hunt and Ramsdell describes morganite as “pale pink to rose red.” Other references use the terms pink, pale violet, apricot, peach, pale yellow-orange, and pale brownish-orange. John S. White states in the Smithsonian Treasury Minerals and Gems that “newly mined specimens of morganite are peach to brownish-orange” and “after prolonged exposure to the sun, the color changes to pink.”
Pictures of two stones from Brazil, a 235.5-carat pink gem donated to the collection by Mr. and Mrs. Frank Ix, Jr., and a brownish-yellow 330-carat gem are shown on page 62 of the volume. Sinkankas notes in his Gemstone and Mineral Data Book that apricot colored material from Brazil altered to pink after a week of exposure to sunlight in San Diego, California. Dr. Richard T. Liddicoat states that much of the pink morganite on the market is yellow to reddish-yellow material from Brazil that has been heat-treated. The color change takes place at 400 degrees centigrade. Mineralogist Walter Schumann observes that the lower color quality material can sometimes be improved by the heat-treatment process.
Miscellaneous facts gleaned from various sources during the research for this article include the following information. Morganite usually has higher refractive indices and a higher birefringence than the other varieties of beryl. In the Color Encyclopedia of Gemstones, Dr. Joel Arem lists the bixbite variety (red beryl) from the Thomas Range in Utah as morganite. Beryl fuses with difficulty to a white pebbly or blister-like material. Fusion causes the loss of its crystalline structure to become singly refractive beryl glass. Simon & Schuster’s Guide to Gems and Precious Stones states that “morganite is not as a rule imitated, nor is it produced synthetically.”
In the July 1995 issue of the New Mexico Facetor, editor Merrill O. Murphy then warned of a glass imitation of morganite that is difficult to detect visually. Synthetic morganite is now being produced by Biros of Australia and was available at the Kimberly booth at the AGTA Show in Tucson this year. These pale peach synthetic gems were quoted at forty dollars per carat.
Richly colored morganite gems are among the more valuable of the secondary gemstones. The value of morganite has increased with the expanding knowledge of the gem-buying public. The more informed consumers become about gemstones, the more it fuels the growing demand for colored gems. Freeform gems, carvings, and new faceted designs join the popular and traditional step-cut rectangle with truncated corners and are appearing in displays more frequently. Beryl’s combination of desirable physical properties and its many color choices make the variety of morganite a gem suitable for all types of jewelry.
|Composition:||beryllium aluminum silicate Be 3 Al 2 Si 6 O 18 + MnCsLi|
|Crystal System:||hexagonal per Arem; hexagonal (trigonal) per Schumann|
|Colors:||pink, rose, pale violet, pale yellow-orange, brownish yellow, reddish-yellow|
|Diaphaneity:||transparent, translucent, semi-translucent|
|Habit:||stubby six-sided prisms, tabular|
|Specific Gravity||varies from 2.60 to 2.90|
|Hardness||7.50 to 8.0|
|Toughness:||very good; can be brittle|
|Refractive Index||o=1.572 to 1.592; e= 1.578 to 1.600|
|Birefringence:||varies from 0.008 to 0.009|
|Optic Character||uniaxial negative|
|Pleochroism||distinct pale pink/bluish pink|
|Luminescence||may fluoresce weak pale violet|
|Color Filter||no information|
|Aqua Filter||no reaction|
|Chelsea Filter||no reaction|
|Solubility||insoluble except in hydrofluoric acid|
|Thermal Traits||color change with moderate heat and light; avoid thermal shock|
|Inclusions||long hollow tubes may contain liquid; veils are common; coarse tubes in pink Brazilian material can cause asterism|