conchoidal

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.

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.

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

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.