披沙揀金

- obtain gold by washing it from sand and gravel; extract sth. of worth from a miscellany; extract sth. valuable from a conglomerate mass; pan gravel for gold; select from a mixture of fine and coarse [of good and bad]; sift sand for gold; sort out the fine gold from the sand -- to extract the essentials from a mass of material; spread out the sand and choose gold; wash gold from gravel or from gold -- to bearing sand