Could you explain the analogy of the stone and the gem that is used to describe a disciple's development?

The analogy of the stone and the gem, found in Chapter 19, illustrates how disciples respond differently to the same spiritual guidance from a Guru. Both a stone and a gem are polished on a whetstone, which represents the Guru's teachings and process. However, their inherent nature dictates the final outcome. The gem, possessing an inner brilliance, becomes a sparkling jewel when polished. The stone, by its nature, can only become smooth; it cannot gain the gem's luster. This analogy suggests that while the Guru provides the same opportunity for refinement to all, a disciple's inner quality—their capacity for faith, patience, and receptivity—determines whether they will transform into a 'brilliant jewel' or merely receive a superficial polish.