Stone that's egg-shaped, soft but not very (4)
I believe the answer is:
opal
'stone' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "little jewel" , "item in Hatton Garden?" , "Translucent gemstone" , "Many-coloured gem" , "Type of gem" .)