Workman in stone (4)
I believe the answer is:
opal
This might be a double definition.
'workman in' is the first definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe there's a link between them I don't understand?
'stone' is the second definition.
(gemstone)
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Semi-transparent gem" , "Milk-coloured gemstone" , "often semi-transparent" , "Delicately coloured gem" , "One in a ring perhaps" .)