An ornamental stone consisting of a hard variety of chalcedony that is typically banded in appearance (5)
I believe the answer is:
agate
(Other definitions for agate that I've seen before include "Old drama critic" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Ornamental form of quartz" , "Stone with coloured bands" , "Variegated chalcedony" .)