Stone jar containing some specimens (6)
I believe the answer is:
jasper
'stone' is the definition.
(I know that jasper is a type of opaque gem)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for jasper that I've seen before include "hard matter" , "Opaque reddish-brown semi-precious stone" , "Reddish-brown stone used in jewellery" , "Opaque quartz" , "stone used by jewellers" .)