Can you explain the analogy of the gem and the stone from the perspective of a disciple's development?

Chapter 19 presents an analogy to illustrate differences in disciple development. It compares a gem and a stone being rubbed on the same whetstone. While both undergo the same polishing process, the outcome is vastly different: "the stone remains a stone, but the gem becomes a brilliant jewel." The stone may become smooth, but it cannot gain the luster of a gem. This metaphor suggests that while a Guru provides the same guidance and process to all, the disciple's inherent nature, receptivity, and spiritual readiness ultimately determine the level of transformation they achieve. It highlights the importance of the disciple's own inner quality.