Can you explain the analogy of the stone and the gem in the context of a Guru's expectations for a disciple?

Chapter 19 presents a powerful analogy to explain how a Guru's guidance interacts with a disciple's inner qualities. It describes how both a stone and a gem are polished on a whetstone. While the process is the same for both, 'the stone remains a stone, but the gem becomes a brilliant jewel.' This illustrates that while a Guru provides teachings and grace to all, the disciple's inherent readiness and qualities, like faith and patience, are crucial for transformation. A disciple with these virtues is like the gem, capable of becoming a 'sparkling diamond,' whereas one without them, like the stone, may be smoothed but will not gain the same spiritual luster.