Can you explain the analogy of the stone and the gem that Sai Baba uses in Chapter 19 regarding a Guru's guidance?

The analogy of the stone and the gem, found in Chapter 19, illustrates the importance of a disciple's inherent qualities and readiness to receive teachings. Baba explains that both a stone and a gem are rubbed on a whetstone, representing the same process of a Guru's polishing guidance. However, the outcome is vastly different: "the stone remains a stone, but the gem becomes a brilliant jewel." This metaphor implies that while a Guru provides the same opportunity for refinement to all, not every disciple will transform. The text suggests that a disciple's "wisdom," "strong faith at the Guru’s feet," and the "strength of courage which is patience" are what allow them, like a gem, to gain luster and become a sparkling diamond. A disciple lacking these qualities, like a stone, remains unchanged despite the process.