conchoidal

1. 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. 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 is distinguished by its splendent lustre and conchoidal fracture.

3. That fractures with planar concentric curves (e.g., as flint, chert or obsidian).

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