Gemstone? Never picked up unknown quantities (4)
I believe the answer is:
onyx
'gemstone?' is the definition.
(onyx is a kind of gemstone)
'never picked up unknown quantities' is the wordplay.
'never' becomes 'no' (I've seen this in other clues).
'picked up' says the letters should be written in reverse.
'unknown quantities' becomes 'yx' (I can't justify this - if you can you should give a lot more credence to this answer).
'no' written backwards gives 'on'.
'on'+'yx'='ONYX'
Can you help me to learn more?
(Other definitions for onyx that I've seen before include "Gemstone, a variety of chalcedony" , "Agate with different bands of colour" , "Semi-precious stone used in making cameos" , "Banded gemstone" , "Form of agate" .)