James A Stone (6)
I believe the answer is:
jasper
'stone' is the definition.
(I know that jasper is a type of opaque gem)
'james a' is the wordplay.
'james' becomes 'jas' (abbreviation for the book of the Bible).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'jas'+'per'='JASPER'
(Other definitions for jasper that I've seen before include "hard matter" , "Reddish-brown stone used in jewellery" , "Precious stone - boy's name" , "Opaque reddish-brown semi-precious stone" , "A man" .)