Precious metals need refinement, a process that heats them up and separates the pure metal from the impurities within it. What about the heart? What purifies it?
The refining pot is for silver and the furnace for gold,
But the LORD tests the hearts.
To refine silver you put it in a pot, which in turn goes into a furnace.
Same thing for gold.
However, what instrument or what process refines the heart? The Lord does it. It may not be pleasant as we go through the fire of refinement, but in the end we surpass silver and gold in value.