idiom · to separate the wheat from the chaff; to extract the best or most valuable parts from a large quantity of material

做学问要有披沙拣金的精神，才能在浩如烟海的文献中发现真理。

to sort the gold from the sand

那个淘金工人在溪流边辛勤地披沙拣金，寻找着细微的金粒。