conchoidal

adjective3

1. •
   Irregular, with planar, concentric curves, similar to those on a mussel shell.

   • It is distinguished by its splendent lustre and conchoidal fracture. — 1821
2. •
   That fractures with planar concentric curves (e.g., as flint, chert or obsidian).
3. •
   Of or pertaining to a conchoid; that may be defined as a conchoid.

   • […] a conchoidal hyperbola with an oval at its convexity. — 1695