Gem of standard weight (5)
I believe the answer is:
stone
This could be a double definition.
'gem' is the first definition.
(I know that gem can be written as gem whale)
'standard weight' is the second definition.
'stone' can be an answer for 'weight' (stone is a kind of weight). I'm unsure of the 'standard' bit.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stone that I've seen before include "See 5" , "Kernel" , "Gem; Staffs. town" , "Boulder" , "Hard lump of mineral matter" .)