Name your old stone, expressing name - this stone? (8)
I believe the answer is:
amethyst
'this stone?' is the definition.
(I've seen this before)
'name your old stone expressing name' is the wordplay.
'your old' becomes 'thy' ('thy' historically meant 'your').
'stone' becomes 'st'.
'expressing' is a deletion indicator.
'name' becomes 'n' (common abbreviation e.g. on forms).
'name'+'thy'+'st'='namethyst'
'namethyst' with 'n' taken away is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "Something precious" , "rock" , "what's in ring" , "Transparent bluish quartz" , "February stone" .)