«إِنَّ خُلَاصَةَ الْجَوْهَرِ تَظْهَرُ بِالسَّبْكِ»
The true quality of a substance appears when it is refined or tested.
Sabk suggests smelting and casting, a process that can reveal the difference between valuable substance and impurity. The stranger compares literary skill to a precious material: its worth is shown by trial, not by self-praise alone.
The connected image of truth breaking into the open strengthens the argument. The metaphors point toward an actual demonstration in the presence of those best able to judge it.