Tectosilicate Mineral Lazurite

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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Hydrothermal vein near Schwartzwalder Mine, Colorado, USA
Courtesy of U.S. Geological Survey, Public Domain

About Thermal Metamorphism

When molten rock (magma) intrudes into the Earth's crust, it may heat up the rocks and minerals that it touches. The temperature may get as high as 700-800 degrees C (1,300-1,450 degrees F), which is still too low to melt the rocks near these igneous intrusions, but is hot enough to change the crystal structure of the minerals that make up these rocks. The resulting metamorphic rocks are known as hornfels. In some places, the heat stimulates the movement of water through tiny openings in the crust, dissolving minerals in some places and redepositing them in other places called hydrothermal veins. For thousands of years, humans have mined these veins for ores of important metals, such as iron, tungsten, tin, uranium, gold, and silver.

Fluorite specimens, different colors, cut as gemstones
Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

The Colors of Minerals

One of the most striking, yet least diagnostic, features of many minerals is their color. Well-formed mineral crystals span the entire rainbow of tinctures, from red (cinnabar, garnet) to yellow (sulfur), green (malachite), blue (azurite, lazurite), and violet (the amethyst variety of quartz). Minerals containing iron and magnesium are often dark brown or dark green. Impurities, trace amounts of elements that do not normally belong in the mineral, may change the overall color of a crystal. For instance, depending on the trace amounts of impurities it contains, quartz may look colorless (no impurities), light pink (titanium, iron, or manganese), milky white (tiny bubbles of gas or liquid), purple (iron), yellow (iron), or brown (extra silicon). However, multiple minerals may have almost the same color, so scientists must rely on other physical properties to make definite identifications of mineral specimens.

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.