According to this stone, Zion included a central circle (6)
I believe the answer is:
zircon
'according to this stone' is the definition.
'zircon' can be an answer for 'stone' (I've seen this before). I am not certain of the rest of the definition.
'zion included a central circle' is the wordplay.
'included' means one lot of letters goes inside another.
'a central' indicates the central letters.
The centre of 'circle' is 'rc'.
'zion' enclosing 'rc' is 'ZIRCON'.
Can you help me to learn more?
(Other definitions for zircon that I've seen before include "Mineral of gem quality" , "Precious stone" , "Gemstone, maybe jacinth" , "Bluish-white gem cut from translucent mineral" , "jargoon?" .)