Conchoidal fracture

A conchoidal fracture is a break or

silica, such as obsidian and window glass, as well as a few metals, such as solid gallium

Crystalline materials such as quartz also exhibit conchoidal fractures when they lack a cleavage plane and do not break along a plane parallel to their crystalline faces. So, a conchoidal, or uneven, fracture is not a specific indication of the amorphous character of a mineral, or a material. Amorphous, cryptocrystalline, and crystalline materials can all present conchoidal fracture when they lack a preferential cleavage plane.

Conchoidal fractures can occur in various materials if they are properly percussed (struck). Cryptocrystalline silica, such as chert, or flint, with this material property were widely sought after, traded, and fashioned into sharp tools in the Stone Age.

Conchoidal fractures often result in a curved breakage surface that resembles the rippling, gradual curves of a

Ancient Greek: κογχοειδής konchoeidēs < κόγχη konchē).^[2]^[3] A swelling appears at the point of impact called the bulb of percussion. Shock waves emanating outwards from this point leave their mark on the stone as ripples. Other conchoidal features include small fissures

emanating from the bulb of percussion.

They are defined in contrast to the

ductile fracture surfaces desirable in most structural applications.^{[citation needed}

]

$Obsidian gives conchoidal fractures.$

Obsidian gives conchoidal fractures.
$Conchoidal fracture in obsidian$

Conchoidal fracture in obsidian
$Conchoidal fracture in flint$

Conchoidal fracture in flint
$Multiple conchoidal fractures in knapped glass$

Multiple conchoidal fractures in knapped glass

Subsets

Several subdefinitions exist, for instance on the Webmineral website:^[4]

Brittle—conchoidal: very brittle fracture producing small, conchoidal fragments
Brittle—subconchoidal: brittle fracture with subconchoidal fragments
Conchoidal—irregular: irregular fracture producing small, conchoidal fragments
Conchoidal—uneven: uneven fracture producing small, conchoidal fragments
Subconchoidal: fractures developed in brittle materials characterized by semi-curving surfaces

Lithics

In

stresses applied, and not by some preferred orientation of the material. This property also makes such fractures useful in engineering, since they provide a permanent record of the stress state at the time of failure. As conchoidal fractures can be produced only by mechanical impact, rather than frost cracking for example, they can be a useful method of differentiating prehistoric

stone tools from natural stones.