Can you explain the analogy of the stone and the gem that Sai Baba uses to describe the effect of a Guru's guidance?

In Chapter 19, Sai Baba uses the analogy of a stone and a gem to explain how different disciples respond to the same spiritual guidance. He notes that both a stone and a gem are polished on a whetstone, representing the uniform process of the Guru's teachings. However, the outcome is different: 'the stone remains a stone, but the gem becomes a brilliant jewel.' This metaphor powerfully illustrates that while the Guru's efforts are consistent, the disciple's inherent nature and readiness determine the result. A receptive and worthy disciple (the gem) will become brilliant, while another (the stone) may only receive a superficial polish without a fundamental transformation.