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Roger Weller, geology instructor            regional geology  planetary  gems


Todd Nehem

Physical Geology

Autumn 2008


Brief Introduction





Brief Introduction to Arrowheads

                Man has been making tools and weapons out of materials found on our planet ever since we started walking over a million years ago. Primitive Man had few resources to work with at the time; some of these were rocks, bones and wood. Out of these three, stone tools became the most durable and withstand time the best. One of the weapons made from stone are points also known as Arrowheads. This paper will talk about some of the materials being used and will give some examples of these materials.

            What makes a great Arrowhead is a hard rock or mineral above 5.5 on Mohs scale of hardness; for example, cryptocrystalline quartz breaks with a sharp conchoidal fracture which is a curved or shell like fracture. Flakes from these fractures can be thin and very sharp. Here are two examples of conchoidal fracturing and flaking, also known as chip or chipping.




Rocks and Minerals-being used








OBSIDIAN (below) is a very fine grained silica-rich volcanic rock with glass as its main component; it is an extrusive igneous volcanic rock that is formed by the very rapid cooling of viscous acid lava. Obsidian is usually black but can range in many different colors like dark olive-green and root-beer smoky; it also can range from transparent to opaque as shown in the examples below. Obsidian is rated 5 to 5.5on Mohs scale of hardness.


MAHOGANY OBSIDIAN (below) in brown- and blacked-streaked ‘mahogany’ and ‘midnight lace’ obsidian (not shown here) swirls and streaks are formed as the cooling lava rolled over and over as iron oxides are mixed in, which gives the obsidian its reddish to brownish colors in the streaks and swirls. Obsidian is the sharpest of all the materials being covered here because it is naturally formed glass and, when chipped, the conchoidal fractures can be very thin and extremely  sharp, like a razor blade.


CHALCEDONY (SiO2 Silicon Dioxide) is when quartz forms at low temperatures in volcanic cavities; its crystals can be so small that the mineral looks like porcelain. Chalcedony is the general name for ‘cryptocrystalline quartz’. It comes in an astonishing array of colors and patterns, including blood-red carnelian, wine-red jasper, brown-banded agate, green-moss agate, apple-green chrysoprase, and black and white onyx. Chert is also a form of chalcedony. 

JASPER (below) Burgundy-wine colored arrowhead, yellow-brown jasper chipping stone.


Yellow-red colored jasper scraper from petrified wood, AZ. 


AGATE (below) Opal agate and a moss-green agate arrowhead


BIOCHEMICAL ROCKS are formed when countless creatures are able to extract dissolved chemicals from seawater and use them to make shell and bone. Some use calcium and carbon to make carbonates, others use dissolved silica to make silicates. When these creatures die, the solid material they create turns into sediments, which form ‘biochemical’ sedimentary rocks such as chert, flint, chalk and diatomaceous earth. Prehistoric man used chert and flint because of the ability to conchoidal fracture, great for making tools with.  Both chert and flint are opaque and have a waxy to glassy appearance.  

CHERT (below) is a form of chalcedony and has a hardness of 7 and above
on Mohs scale of hardness. Chert is a light tan to brown in color.


FLINT (below) is the replacement for chert and ranges from dark grey-brown to black.


            Obsidian, jasper, agate, chert, and flint are not the only materials that prehistoric man

used to make tools and weapons with. They also used hornfels, rhyolite and anthracite.


Works cited

Farndon, John. The Complete Guide To Rocks and Minerals. London: Hermes House, 2006.
Schumann, Walter. Gemstones of the World. New York: Sterling, 1997.
Photographs of arrowheads by Roger Weller

Pellant, Chris. Rocks and Minerals: The visual guide to more than 500 rocks and minerals from around the world.
Ed. Helen Pellant. New York: Dorling Kindersley, 1992.