Sedimentary Rock Chert

Associated Smithsonian Expert: Leslie Hale, B.S.

Leslie Hale at the Monterey Bay Aquarium (California, USA) during a conference of museum collections managers

Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

Leslie Hale, the Smithsonian’s rock and ore collections manager at the Smithsonian National Museum of Natural History, tells people that she is a “rock librarian.” While growing up in Bowie, Md., not far from the Smithsonian, Hale collected rocks and took a summer class on lapidary art (making jewelry out of stone). Her career choice was greatly influenced by her attendance at a magnet high school for science and mathematics and taking geology as a senior-year elective. Hale joined the Smithsonian staff shortly after finishing her bachelor’s degree at the University of Maryland in 1989. Today, she supervises two full-time staff members as well as several volunteers, interns, and contractors. She assists Smithsonian scientists and visiting researchers who want to use the museum’s extensive collection of rock and ore specimens. Hale also sends out rocks on loan to geologists at distant universities; she keeps track of the objects’ whereabouts and sends out requests for return or loan renewal. Finally, she gives tours of the Smithsonian’s geology research facilities, conducts inventories, answers questions from the public, and identifies rock specimens.

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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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Deformed mudstone, Fox Island, Alaska, USA
Courtesy of U.S. Geological Survey, Public Domain

About Silt and Mud in Sedimentary Rocks

Silt grains are rock particles even smaller than sand (less than 0.06 mm or 0.002 inch wide), so water and wind can transport them farther than sand grains. When the water in a pond appears brown and difficult to see through, it may be carrying large amounts of silt, which is slow to settle to the bottom. Quartz and feldspar, both silicates, are the most common minerals in silt. Once silt hardens into rock, scientists usually cannot see the individual grains without the aid of a microscope. Mud is a mixture of water, silt, soil, and clay (the tiniest of all rock particles). Mudstone, the sedimentary rock made of mud, often breaks into random blocks, and it may contain cracks that first appeared when the original mud was drying out.

Ancient stone tools showing the pace of remarkable technological enhancements over time (1.75 to 0.85 million years ago)
Courtesy of Los Alamos National Laboratory

Prehistoric Stone and Metal Tools

More than 2 million years ago, early humans began to strike stones against each other to shape them into the first tools. Early toolmakers used chert, quartzite, basalt, obsidian, and similar rock types because their crystalline structure gives the stone tools their sharp edges. About 7,500 years ago, humans in southeastern Europe learned how to melt native copper and cast it in molds to make tools and ornaments. About 6,000 years ago in North America, Native Americans were also making tools and ornaments from copper that came from rich deposits in the Lake Superior area. Next, people discovered how to extract copper and other metals from rocks by crushing the rocks and sorting out the bits of metal. The bits were then smelted to create a pure metal. People also noticed that by combining different metals, they could make a new one that might be stronger or a different color. When copper is mixed with tin the result is bronze, which is much harder than pure copper or tin and has a more golden color. More than 5,000 years ago, humans in Asia made a few ornamental items from iron-nickel meteorites - the only kind of native iron available. Only about 3,300 years ago did humans figure out how to smelt iron from ore. The first iron that was produced was soft, but when carbon was added to the iron it produced steel, which is much harder. The downside was that there was little control over how to get the right balance of carbon and iron, and the steel could be brittle. The Romans solved this problem by "piling" or layering steel and iron and then forging and quenching it to make the metal durable and less brittle. In 200 CE (Common Era), people improved the process by pattern welding, taking rods of steel and twisting them together and then hammering them into a flat sheet.