The Best Rock Used for Tools, Before and After the Discovery of Metal

Use of stone

Most people know that stone “arrowheads” were made from a kind of rock called flint, but otherwise
have no idea about the relationship of stone as a material to tool manufacture and use. Early hominids were more discerning; they had to be to survive.




Flint

The best material for making a great variety of stone tools, flint is closely related to the semiprecious stones called carnelian, chrysoprase, and jasper—uniform, red, green, or yellow forms of the mineral chalcedony. Large deposits of gray or black chalcedony are called chert; small pieces, called nodules, of gray or black chalcedony found in limestone or chalk are called flint. The harder and more chemically stable flint can easily be picked from its limestone or chalk background.

Limestone, chalk and Quartz

No one knows what causes flint nodules to appear where they do. Chalcedony is a form of quartz that has tiny crystals and is very dense; hence, it is silicon dioxide, also known to mineralogists
as silica, just as quartz is. Limestone and chalk are both calcium carbonate, a different mineral entirely. Veins of silicon dioxide often form as a result of solutions of water containing the silica, especially solutions in superheated water. The solutions travel through limestone or chalk and leave the silica behind when they cool.

Crystal structure

Because of its crystal structure, flint breaks in a pattern geologists call a conchoidal fracture; this produces sharp edges but does not propagate throughout a stone, splitting or shattering it. Furthermore, there are no preferential fracture planes, so small pieces of almost any shape can be removed. A nodule broken in two could be the first manufactured stone tool, indistinguishable as a tool today except for microscopic wear patterns that indicate use. The beautifully scalloped surfaces of many later stone tools are one result of the conchoidal fracture pattern. In the absence of the fine-grained flints, our ancestors often used the best approximations they could find –– quartz (silicon dioxide with a larger crystal structure) and rocks or other materials infused with silica, including petrified wood.




Obsidian

Anyone who has picked up the remains of a shattered glass dish or bottle knows that glass also breaks in a pattern that causes very sharp edges. The break is also a conchoidal fracture, but because glass is more brittle than flint, the glass not only fractures but also easily shatters. A fragment of broken glass can have a very sharp cutting edge that can be used as a tool. The utility of broken glass was not lost on our ancestors. Although manufacture of glass from silicon dioxide (sand that is formed from small particles of quartz) did not start as far as we know until about 1400 BCE, implements made from natural glass called obsidian are among the earliest stone tools. Obsidian is a rock produced when granite or rhyolite, quartz combined with feldspar and mica, is melted by a volcano and then cooled very quickly. Some dark forms of obsidian are known as pitchstone. Opal, a glassy form of quartz, was also used for stone tools, but it is less common than obsidian. As with substitutes for flint, volcanic rocks such as lavas or those formed from hot
ash flows were sometimes used instead of volcanic glasses.

Other rocks

A third type of stone tool material includes various rocks, such as quartzites or hardened shales, that had been hardened (metamorphosed) by great heat and pressure in the interior of Earth. Quartzites that are metamorphosed sandstones became particularly useful for axe and adz heads after the practice of grinding edges was introduced as part of the Neolithic Revolution. Quartzite axe heads have the advantage of a structure in which cracks do not propagate far, so the tools maintain their integrity even when their edge is chipped. Very early stone bifaces (hand axes) were also made from quartzite, which can be easily flaked to produce a useful but not very attractive tool. As our ancestors became more experienced, they first shifted to the volcanic lavas that could yield smaller, better looking flakes with sharper edges. Eventually, flint came to predominate, even in regions where flint is not a common rock.

Flint and obsidian mining and trading

Flint and obsidian were mined and traded in the later Stone Age. As population increased and there was less flint to go around, new techniques, such as manufacture of microliths, were developed. Microliths involve flint or obsidian cutting edges embedded in wood, with the bulk of the tool embodying the wooden portion.




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