Stone jar containing some specimens (6)

I believe the answer is:

jasper

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'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" .)

I've seen this clue in the Evening Standard.

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