Sentence:  If grains derived from these rocks are unweathered after separation from the parent rock (a very rare occurrence), they may contain either <WSD>conchoidal</WSD> breakage patterns if grains are large, or upturned plates and flat upper and lower surfaces if small grains are considered. 
Meaning of the word marked with <WSD> tags:  Irregular, with planar, concentric curves, similar to those on a mussel shell.