Stone ring with occasional pearls (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'ring with occasional pearls' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'with' means one lot of letters go next to another.
'occasional' indicates alternate letters (letters taken at regular intervals or on occasion).
The alternate letters of 'pearls' are 'pal'.
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Shimmering stone" , "Jewellery item" , "Glassy gemstone" , "Gem stone" , "Milk-coloured gemstone" .)