Geology Home Page physical geology historical geology
Roger Weller, geology instructor regional geology planetary gems
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.
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.
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
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.
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.
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.