con·choi·dal 
(kŏng-koidl)
(kŏng-koidl)adj.
Of, relating to, or being a surface characterized by smooth, shell-like convexities and concavities, as on fractured obsidian.
[From Greek konkhoeidēs, mussellike : konkho-, concho- + -oeidēs, -oid.]
con·choidal·ly adv.