Can you explain the analogy of the stone and the gem that Sai Baba uses to describe the disciple's role in their own spiritual development?

Sai Baba uses an insightful analogy in Chapter 19 to explain the importance of a disciple's readiness. He compares a stone and a gem being rubbed on the same whetstone, which represents the Guru's guidance. While both undergo the same process, only the gem becomes a brilliant jewel; the stone merely becomes smooth. This illustrates that the Guru's teachings, while universally applied, produce different results based on the disciple's inherent nature and spiritual preparedness. A disciple with the qualities of a 'gem,' such as faith and patience, can be polished into a sparkling diamond, whereas one who is like a 'stone' cannot gain the same luster, implying that the disciple's own qualities are crucial for transformation.