Native Californians traditionally used a variety of natural materials for making tools, clothing, jewelry, and other items:  stone, bone, shell, wood, plant fibers, sinew, feathers, and the like.  Most of these items are highly perishable and do not survive well in the archaeological record.  The exceptions are bone, shell, and especially stone.  In fact, stone – flaked, ground, battered, and otherwise modified – makes up more than 90% of most archaeological assemblages in California.

The kind of stone an artifact is made of can tell us something about the movements and interactions of prehistoric people.  For instance, an arrow point made of Napa Valley obsidian that is found at a site in the Sierra tells us that people, or trade goods, were moving back and forth between those areas.   The same can be said of a handstone made of quartzite that is found in the Sacramento Valley, where no quartzite occurs naturally.

Some rock basics

Rocks are made up of various minerals.  The kinds of minerals, and their proportions, determine the colors of the rocks. For instance, granite is made up largely of quartz (white), feldspar (pink), and the dark minerals hornblende and mica.  Basalt, on the other hand, has mostly dark minerals (pyroxene, olivine, calcium-rich plagioclase), so it is dark or very dark grey.  These are the two most prevalent types of rock in California – in fact, basalt is the most common rock on the planet.  High-quality basalt suitable for stone tools, however, is more rare.

The structure and texture of rocks is due in large part to how big the minerals are; the faster the rock cooled when it was being “born,” the less time the minerals had to grow.  Thus, basalt, which oozes from the ground and cools fairly quickly (when it hits air or water), has very tiny minerals that aren’t easily seen with the naked eye.  We refer to this as a fine-grained or aphanitic material.  Granite forms beneath the surface and cools much more slowly, so the minerals have time to grow large – large enough for us to see them without magnification.  Granite is a coarse-grained or phaneritic rock.

A geologist will tell you that the only sure way to identify a rock is to cut a thin-section and look at it under a microscope.  Since that’s not practical for the field (and most non-geologists – including most archaeologists – wouldn’t know what they were looking at, anyway), it’s best to be somewhat general when describing raw materials.  We can note easily if they are fine-grained or coarse-grained, what color(s) they are, and if they contain identifiable minerals or inclusions.  It’s not necessary to know the difference between, say, granite and diorite – it’s enough to say “coarse-grained igneous or granitic rock.”    

Fine-grained materials are best for flaked-stone tools (projectile points, bifaces, etc.), because they fracture in more predictable and controllable ways.  Coarse-grained rocks often are used for grinding stones, since the coarseness helps in the grinding process, but they are seldom if ever used for projectile points or other flaked-stone implements.

Here are the basic rock “families” recognized by most geologists:

Common types of tool stone found in California sites

Flaked-stone implements are often made of obsidian, cryptocrystalline silicate stone (CCS), slate, or fine-grained basalt.

				Napa Valley obsidian Obsidian nodule with exterior cortex (left) Flake of obsidian removed from nodule (center) Obsidian biface (right); note flaking pattern
	Obsidian comes in different colors (black, grey, red, green, etc.) and textures (frothy, “sugary,” rough, glassy-smooth).  Some is completely opaque, some cloudy or banded, and some nearly translucent.  These visual characteristics can help identify the quarry where the obsidian was collected.

				Fine-grained basalt from Gold Lake, Plumas County Nodule with exterior cortex (left) Flake tool (right); note flaked edges
					Granite
						Franciscan and Monterey cherts (left and below) A very common type of CCS, chert, comes in a variety of colors.  Monterey chert from the central California coast is black and sometimes has lighter bands.  Franciscan chert from the North Coast Ranges is green or red.  Chert from the Sierran foothills can be many colors – yellow, brown, red, white, green – often with two or more colors on the same artifact.  Chert tool stone often was heated first, to improve its flaking qualities.  Heat-treatment can change both the color and the texture of CCS stone, sometimes giving it a “waxy” appearance.

	Vesicular volcanic material (vesicles formed by gas bubbles)

		Quartzite cobble (note fine-grained texture); extremely hard material suitable for hammerstones and other tools used for heavy battering

		“Soapstone” or steatite; very soft, easily scratched or carved (often made into bowls or vessels)

	Quartz crystal (left) and milky quartz artifacts (right); extremely hard material, very difficult to make into artifacts

Other raw materials

Although they are more perishable (and thus are not found as often), some artifacts were made of wood, bone, shell, and other organic materials.  Any bone fragments found at an archaeological site should be examined closely to see if they are modified (ground, polished, cut, incised, etc.).  Bone at a site also might reflect diet (deer, rabbit, bird, fish), and sometimes it signals the presence of human burials.  It is important to differentiate between fresh or “green” bone – say, from a deer that died the previous winter – and older, drier, highly weathered bone.  Fresh bone will be much heavier and much smoother to the touch than archaeological bone.  Also look for tell-tale signs of modern butchering (“t”-bones, ham bones, and the like).

		Fresh or “green” bone (left) Modern butchering (center) Older, weathered bone (right)
			Bone awl

Fragment of abalone shell (left) Clam shell (center) Beads made of Olivella shell

Though they are somewhat rare, we do find arrow points and flake tools of man-made glass, like the specimens you see here.  Obviously we can date these to the Historic period, since pre-Contact native people in California did not make glass.

Prehistoric pottery is rare in most of California (most tribes used baskets, instead), but it does occur in the far southern areas of the state, especially in the southeastern deserts and along the coast.  Some baked-clay items have also been collected from sites in the Sacramento area.  Note the obvious shaping of these pieces, especially the rim fragments (on the far left and far right).

Side view of rim piece.

Side view of rim piece.

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