• (a.) Having elevations or depressions in form like one half of a bivalve shell; -- applied principally to a surface produced by fracture.
Shaped like a shell; having alternate convexities and concavities on the surface. ... Origin: concha + G. Eidos, appearance ... (05 Mar 2000) ... 
Con·choid'al adjective [ Confer French conchoïdal .] (Min.) Having elevations or depressions in form like one half of a bivalve shell; -- applied principally to a surface produced by fracture. 
Type: Term Pronunciation: kon-koy′dăl Definitions: 1. Shaped like a shell; having alternate convexities and concavities on the surface.
conchoidal 1. Used to describe fracture surfaces in rocks and minerals that are concave or convex and shaped like shells. 2. Having or being a surface shaped like a bivalve shell with smooth ridges and depressions; such as, conchoidal fracture. 3. Relating to, or being a surface characterized by smooth, shell-like convexities and concavities, as...
smooth, concave, shell-like fractures characteristic of, for example, glass and quartz. 
A distinctive fracture pattern and one displayed by broken glass. The mineral breaks with a concave or convex surface often showing concentric ridges looking like growth lines on a shell.
noting a shell-like fracture form produced on certain minerals by a blow.