Stone plaza is rocky you said, piercing bad back (5,6)
I believe the answer is:
lapis lazuli
'stone' is the definition.
(I've seen this before)
'plaza is rocky you said piercing bad back' is the wordplay.
'rocky' indicates anagramming the letters (letters rocked or thrown around).
'you said' becomes 'U' (homophone of 'you').
'piercing' is an insertion indicator (some letters pierce a hole in a word).
'bad' becomes 'ill' (similar in meaning).
'back' shows that the letters should be reversed in order.
'ill' backwards is 'lli'.
'plaza'+'is'='plazais'
'plazais' is an anagram of 'apislaz'.
'apislaz'+'u'='apislazu'
'apislazu' put within 'lli' is 'LAPIS-LAZULI'.
(Another definition for lapis lazuli that I've seen is " Bright blue stone".)