C. F. Keil and Franz Delitzsch on Proverbs 17:3

Pro 17:3 3 The fining-pot for silver, and the furnace for gold;    And a trier of hearts is Jahve.

An emblematical proverb, which means that Jahve is for the heart what the smelting-pot (from צרף, to change, particularly to melt, to refine) is for silver, and what the smelting furnace (כוּר, from כּוּר, R. כר, to round, Exo 22:20) is for gold, that Jahve is for the heart, viz., a trier (בחן, to grind, to try by grinding, here as at Psa 7:10) of their nature and their contents, for which, of the proof of metals, is elsewhere (Pro 16:2; Pro 21:2; Pro 24:12) used the word (cf. בּחון, the essay-master, Jer 6:7) תּכן, weigher, or דּורשׁ, searcher (1Ch 28:9). Wherever the subject spoken of is God, the searcher of hearts, the plur. לבּות, once לבבות ecno ,, is used; the form לבבים occurs only in the status conjunctus with the suffix. In Pro 27:21 there follow the two figures, with which there is formed a priamel, as at Pro 26:3, another tertium comparationis.