Fentanyl forms dark branched comb or feathery clusters with the aqueous gold chloride. In acidic conditions, the crystal forms get smaller. In aqueous conditions, the crystals grow very long and highly branched. The formations are very bright under crossed polars and more details are visible. In light of the safety issues associated with fentanyl, handling this test with powders is not advised. However, these results are given to caution analysts who may encounter these formations as part of mixed street samples.