Friend covered with very large gems (5)
I believe the answer is:
opals
'gems' is the definition.
(I know this)
'friend covered with very large' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'covered with' indicates putting letters inside.
'very large' becomes 'os' (abbreviation for oversize).
'pal' going inside 'os' is 'OPALS'.
(Other definitions for opals that I've seen before include "Milky-white stones" , "Translucent gemstones" , "jewellery" , "jewels" , "Milky white and coloured gems" .)