Hard sedimentary rock (9)
I believe the answer is:
ironstone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hard sedimentary rock</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ironstone that I've seen before include "Pottery" , "Mineral" , "Material" , "Sort of rock" , "Earthenware" .)