View source for Conchoidal fracture

{{Short description|Brittle fracture surface that does not follow any natural planes of separation}}

[[File:Lipari-Obsidienne (5).jpg|thumb|Conchoidal fractures in [[Obsidian]]]]
A '''conchoidal fracture''' is a break or [[fracture]] of a [[brittle]] material that does not follow any natural [[cleavage (crystal)|planes of separation]]. [[Mindat.org]] defines ''conchoidal fracture'' as follows: "a fracture with smooth, curved surfaces, typically slightly concave, showing concentric undulations resembling the lines of growth of a shell".<ref name=MindatConchoidal>[https://www.mindat.org/glossary/conchoidal_fracture Conchoidal fracture] at [[Mindat.org]]</ref> Materials that break in this way include [[quartz]], [[chert]], [[flint]], [[quartzite]], [[jasper]], and other fine-[[crystallite|grain]]ed or [[amorphous solid|amorphous]] materials with a composition of pure [[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 (crystal)|cleavage plane]] and do not break along a plane parallel to their crystalline faces. Hence, a conchoidal or [[fracture (mineralogy)|uneven fracture]] is not a specific indication of the amorphous character of a mineral, or a material. [[amorphous solid|Amorphous]], [[cryptocrystalline]], and [[crystal]]line materials can all present conchoidal fracture when they lack a preferential cleavage plane.

Conchoidal fractures can occur in various materials if they are properly [[wikt:percuss|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 [[mussel]] shell; the word "conchoid" is derived from the word for this animal ({{langx|grc|κογχοειδής}} ''konchoeidēs'' < {{lang|grc|κόγχη}} ''konchē'').<ref>{{cite web |url= https://www.merriam-webster.com/dictionary/conchoidal |title= Conchoidal |accessdate= 2010-10-30 |work= Merriam-Webster Online}}</ref><ref>{{LSJ |kogxoeidh/s|κογχοειδής}}, {{LSJ|ko/gxh|κόγχη|ref}}.</ref> 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 [[wikt:fissure|fissure]]s emanating from the bulb of percussion.

They are defined in contrast to the [[facet]]ed fractures often seen in single [[crystal]]s such as semiconductor [[wafer (electronics)|wafers]] and [[gemstone]]s and to the high-energy [[ductile]] fracture surfaces desirable in most structural applications.{{Citation needed|date=November 2013}}

<gallery widths="200px" heights="120px" class="center">
File:2017-Obsidian-conchoidal-fracture.jpg|Conchoidal fracture in [[obsidian]]
File:Flintasten.JPG|Conchoidal fracture in [[flint]]
File:Conch fract glass.jpg|Multiple conchoidal fractures in [[knapping|knapped]] [[glass]]
</gallery>

==Subsets==
Several subdefinitions exist, for instance on the Webmineral website:<ref name=WebminderalFracture>[http://webmineral.com/help/Fracture.shtml Mineral Tenacity and Fracture] at Webmineral</ref>

* 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 [[lithic flake|lithic]] [[stone tool]]s, conchoidal fractures form the basis of flint [[knapping]], since the shape of the broken surface is controlled only by the [[stress (physics)|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]].{{citation needed|date=March 2025}} As conchoidal fractures can be produced only by mechanical impact, rather than [[frost]] cracking for example, they can be a useful method of differentiating [[Stone Age|prehistoric]] stone tools from natural stones.{{citation needed|date=March 2025}}

==See also==
{{Portal|Geology}}
* [[Fracture (mineralogy)]]

==References==
{{Reflist}}

==External links==
{{Commons category}}
* {{Wiktionary-inline|conchoid}}

[[Category:Lithics]]
[[Category:Materials degradation]]
[[Category:Mineralogy concepts]]
[[Category:Petrology]]
[[Category:Stone Age]]