Gemstone ring is with friend (4)
I believe the answer is:
opal
'gemstone' is the definition.
(opal is a kind of gemstone)
'ring is with friend' is the wordplay.
'ring' becomes 'o'.
'is with' means one lot of letters go next to another.
'friend' becomes 'pal' (pal is a kind of friend).
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Jewellery item" , "Milky white gemstone" , "item in Hatton Garden?" , "Birthstone for October" , "Form of hydrated silica, often used as a gemstone" .)