Gem in ring presented to friend (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'ring presented to friend' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'presented to' means one lot of letters go next to another.
'friend' becomes 'pal' (pal is a kind of friend).
'o'+'pal'='OPAL'
'in' acts as a link.
(Other definitions for opal that I've seen before include "Jewellery item" , "Precious stone" , "Form of hydrated silica, often used as a gemstone" , "Gem stone" , "One in a ring perhaps" .)