You can only get one grain per piece deconstructed if you are successful, the only thing you can do is increase your odds of success.
For extracting grains successfully from jewelry pieces you need the passive "Extraction" at level 3 (max level).
FYI, even with the extraction passive at the max level players don't have guaranteed drops.
To have higher chances of getting grains from daily writs rewards your jewelry crafting rank needs to be level 5 (max level).
Edited by redlink1979 on September 25, 2019 12:58PM
"Sweet Mother, sweet Mother, send your child unto me, for the sins of the unworthy must be baptized in blood and fear"
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