Stone boulder apparently fairly close, every second counting (4)
I believe the answer is:
opal
'stone' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Gem stone" , "Delicately-coloured gem" , "Shimmering stone" , "Type of gemstone" , "Milky stuff that's hard" .)