Porous gemstone (4)
I believe the answer is:
opal
'porous gemstone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opal that I've seen before include "it may be highly valued" , "Milky-white gemstone with flashes of colour" , "Many-coloured gem" , "Milky white gemstone" , "Precious stuff" .)