English

A conchoidal fracture of a piece of obsidian.

Adjective

conchoidal ‎(comparative more conchoidal, superlative most conchoidal)

1. (mathematics) Of or pertaining to a conchoid; that may be defined as a conchoid.
   • c. 1695, Isaac Newton, 1976, D. T. Whiteside (editor), The Mathematical Papers of Isaac Newton, Volume 7: 1691-1695, page 621,

     […] a conchoidal hyperbola with an oval at its convexity.

2. (mineralogy, of a fracture) Irregular, with planar, concentric curves, similar to those on a mussel shell.
   • 1821, Coal, entry in Andrew Ure, A Dictionary of Chemistry on the Basis of Mr Nicholson's, unnumbered page,

     It^[pitch-coal] is distinguished by its splendent lustre and conchoidal fracture.

   • 2005, Chapter 19: Lithic Studies, Herbert D. G. Maschner, Christopher Chippindale (editors), Handbook of Archaeological Methods, Volume 1, page 719,

     Some flakes detached in a controlled manner have characteristics of conchoidal fracture.

   • 2011, Robert C Shaler, Crime Scene Forensics: A Scientific Method Approach, page 479,

     On one side of the radial edge, the conchoidal fractures form right angles, that is, form a perpendicular with the edge.

   • 2011, David H. Krinsley, John C. Doornkamp, Atlas of Quartz Sand Surface Textures, page 10,

     If grains derived from these rocks are unweathered after separation from the parent rock (a very rare occurrence), they may contain either conchoidal breakage patterns if grains are large, or upturned plates and flat upper and lower surfaces if small grains are considered.

3. (mineralogy, of a mineral) That fractures with planar concentric curves (e.g., as flint, chert or obsidian).
   • 1821, Focus of Philosophy, Science and Art, Volume 1, Issues 1-4, page 74,

     The customary transitions of pitchstone are, into Chalcedony, Chert, Semi-opal; also into a substance resembling conchoidal shining jasper, found in clay strata, entangled in trap, or volcanic rock; and lastly, into Trap.