Cyclosilicate Mineral Beryl

Associated Smithsonian Expert: Jeffrey E. Post, Ph.D.

Jeffrey Post

Photograph by Cara Santelli, Smithsonian Institution

Dr. Jeffrey Post is the curator of the National Gem and Mineral Collection at the Smithsonian National Museum of Natural History. As far back as he can remember in childhood, Post collected rocks and fossils around his home near Madison, Wis. The symmetry of mineral crystals fascinated him, and experiments with a large chemistry set helped develop his interest in science. He earned a Ph.D. from Arizona State University in 1981 and joined the Smithsonian in 1984. Post’s research projects include the physical and chemical properties of fine-grained, environmentally significant minerals such as clays, manganese oxides, and iron oxides. He also uses powerful X-ray beams at the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, N.Y.) to study the crystal structures of these minerals. With his Smithsonian colleagues, Post is always seeking new gem and mineral acquisitions for the Smithsonian. He analyzes specimens to resolve curatorial questions, oversees loans of Smithsonian gems to other museums, supervises the team that is building the collection website, meets with donors, and answers public inquiries about the Smithsonian mineral collection.

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This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Model of corundum's hexagonal crystal structure (red represents oxygen; silver represents aluminum)
Courtesy of Special Collections & Archives Research Center, Oregon State University Libraries

About Chemical-Compound Minerals

Most minerals are composed of two or more elements with their atoms arranged in a regular structure, called a crystal lattice or crystal structure. In chemistry, this is known as a compound. To visualize crystal structure, think of balls attached to other balls with sticks to make regular three-dimensional patterns. The balls represent atoms and the sticks represent the forces between the atoms. Electromagnetic forces between atoms hold some chemical compounds together. In other compounds, the atoms form covalent bonds, which means they share electrons. Scientists classify many minerals into groups based on the types of atoms found in the minerals. For example, oxides consist of metallic atoms bound to oxygen atoms, and sulfides are combinations of metal and sulfur atoms.

Magma rich in silica was injected into cracks in the older gneiss, creating this silicic intrusion, mainly consisting of pink feldspar and white quartz at its core. Location: Morrison, CO.
Photographed by Donald E. Hurlbert, Smithsonian Institution

Crystals and Pegmatites

Pegmatites are extremely coarse-grained intrusive igneous rocks, containing crystals that are both large (at least 5 cm or 2 inches across) and packed closely together. Pegmatites crystallize during the final stages of granite formation. The same silicate minerals that form granite - quartz, feldspar, and mica - generally make up the bulk of pegmatites, too, but the individual minerals in pegmatites can be many centimeters or even several meters in diameter. Some pegmatites also contain less common minerals, such as garnet, albite, lepidolite, beryl, and fluorite. Geologists and miners sometimes find beautiful, gemstone-quality crystals of topaz, beryl (aquamarine), rose quartz, smoky quartz, and other minerals within pegmatites.

The Rosser Reeves star ruby, a corundum specimen from Sri Lanka cut as a 138.7-carat oval cabochon
Photo by Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution

Gemstones and Other Ornamental Stones

Gemstones are minerals that are cut and polished to make beautiful "stones" for jewelry and other adornments. Humans traditionally consider diamonds, rubies, sapphires, and emeralds to be the four "precious" gemstones, with all other gemstones called "semiprecious." Many gems are transparent, meaning they allow some or most of the incident light to pass through them. To make a stone sparkle, gemcutters cut and polish the facets, or flat outer surfaces, of a gemstone to boost the number of times a light beam will reflect internally before leaving the stone and reaching the eye. Some semiprecious stones, such as malachite and lapis lazuli (a rock containing the mineral lazurite), are opaque to light; gemcutters polish them to a high gloss, without facets. This shiny, rounded type of gemstone is called a cabochon. Corundum (ruby or sapphire) specimens that contain tiny fibers of other minerals also may be cut into cabochons to create a luminous "star" effect on the stone's surface.