Boyd’s Bible Dictionary:
(who makes fine). A worker in precious metals (Isa. 1:25; Isa. 40:10; Jer. 6:29; Zech. 13:9; Mal. 3:3).
Concise Bible Dictionary:
God is the refiner of His people, as the precious metals have to be separated from the dross that clings to them (Prov. 25:4—where the AV has “finer”; Isa. 48:10; Zech. 13:9; Mal. 3:2-3). So God tests the believer’s heart (1 Thess. 2:4), and his faith, which leads to endurance (James 1:12; 1 Pet. 1:7).