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Conchoidal fracture: A lucky break

Conchoidal fracture: A lucky break

Conchoidal fracture was once described by an anthropologist as the first big step in human technological development — a “lucky break.” She was referring to the serendipitous discovery some 2.5 million years ago that certain high-silica stone materials would fracture conchoidally and could be broken, or flaked, into sharply edged and pointed tools and weapons. The flint-knappers still use controlled, systematic conchoidal fracture, which was developed by Paleolithic Stoneworkers.

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Mineral cleavage (separation along internal crystal planes) is not the same thing as fracture. Instead, fracture is the irregular breaking up of rocks and minerals. When struck by a hard object, all fine-grained and brittle minerals, including flint will fracture. There are many types of fractures, including uneven, hackly and irregular.

Conchoidal Fracturing

Conchoidal fracture produces smooth, concave surfaces that resemble the interior surfaces of bivalve shells. The word “conchoidal” comes from the Greek konchoeidēs, meaning “like a mussel.”

Conchoidal fracture is seen mainly in minerals and rocks with poor cleavage. It can also be found in some homogeneous noncrystalline materials. Conchoidal cracks can be easily recognized as a characteristic of certain minerals.

Conchoidal Fracture: Mechanics

Modern knappers have carved these beautiful points using a colorful piece of flint.
Wade Wilcox

Conchoidal fracturing occurs at impact. It is accompanied with primary and secondary shockwaves of energy which radiate throughout the material. The direction of the impact determines the fracture direction.

The shock waves are emitted at a slight angle from the surface when a knapper strikes an object made of flint. A part of the energy from shock waves is immediately absorbed by fracturing. The shock waves will then steer the fracture toward the surface as the energy of the shock wave decreases. A conchoidal depression is left on the flint surface when an impact with the right energy, duration, and direction displaces the flake.

Conchoidal depressions are often marked with concentric, curvilinear ridges called “Wallner lines” that are perpendicular to the direction of fracture progression. These growth rings are caused by the interaction of secondary and primary shock waves.

The conchoidally flaked osidian is extremely thin, and can be remarkably sharp.
Steve Voynick

Obsidian & Pyrite

Many knappers believe that obsidian is the ultimate flaking materials. Obsidian, a volcanic glass, is a noncrystalline, homogeneous mineraloid that has a conchoidal fracture. The thinness and sharpness its conchoidal flake is what makes it so special. From Aztec sacrificial blades to modern scalpels, flaked obsidian is used for everything.

In contrast to obsidian pyrite exhibits both conchoidal as well as irregular fracture characteristics. However, the tendency of pyrite’s fracture to occur conchoidally makes it difficult to flake into useful shapes.

Gem Materials

Conchoidal fractures can occur in some gems. Opal is a mineraloid composed of high silica and non-crystalline structure, similar to obsidian. It is fragile and can easily fracture into conchoidal cracks.

Peridot is also a gem that can fracture conchoidally, despite having well-developed planes of cleavage. Conchoidal fractures can occur in transparent colored gems. This is most commonly seen as small chips around the gem’s girdle.

Amber is prone to conchoidal cracking at cold temperatures. Jet, which is a compact black lignite in gem form, is susceptible to the same conchoidal fractures that amber exhibits.

The quartzite used to make these projectile points dates back 8,000 years.
Wikimedia commons


Conchoidal fractures can occur in even very fine-grained rock such as rhyolite (a volcanic extrusion rich in silica) and quartzite (a metamorphosed, metamorphosed, sandstone).

Paleolithic stoneworkers used flaked rhyolite, quartzite and obsidian extensively, despite the fact that their edges and points are not as sharp as those in flint and obsidian. Their abundance made them vital for the production and use of tools and weapons in areas where flint, obsidian, or both were scarce. Rhyolite or quartzite lacks the highly visible, glassy surface of flint or obsidian. They are therefore rarely used to knapp.

Conchoidal crack is commonly thought of as the dozens, or even hundreds, of small concave depressed on the surface of flaked stones weapons and tools. These fractures are sometimes huge. An example is the 60-foot-wide conchoidal depression in a silicified sandstone cliff at Utah’s Arches National Park that is believed to have formed from the impact of a large, falling boulder.

Conchoidal fracture in Archaeology

Because conchoidal fractures form only by mechanical means and never by “frost-cracking” or other tresses, they enable archaeologists to differentiate stone artifacts from similarly appearing, natural objects.

Obsidian’s unusual weathering properties help archaeologists to date flaked artifacts. After flaking the obsidian, the newly exposed surfaces slowly hydrate, altering into perlite. A thin hydration layer is formed that can be precisely measured. The thickness of the hydration rind is independent of climate conditions and directly reflects the age of artifacts made of flaked obsidian.

The story of human evolution is told by the carved stone that was once vital for human survival. It’s also the most durable, abundant and long-lasting of all Stone Age artifacts. Today, knapping continues to be an art. Flaking stone has quite a legacy, all of it because of our ability to exploit conchoidal fracturing, a skill that began, as that anthropologist so astutely said, with a “lucky break.”

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The post Conchoidal Fracture: A Lucky Break first appeared on Rock & Gem Magazine.

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