What is the meaning of the analogy of the stone and the gem in Chapter 19?

The analogy of the stone and the gem, found in Chapter 19, illustrates the importance of a disciple's inherent qualities in the process of spiritual development. The text explains that both a stone and a gem can be rubbed on a whetstone, which represents the guidance and teachings of the Guru. However, while the gem becomes a 'brilliant jewel' and a 'sparkling diamond,' the stone merely becomes smooth but remains a stone. This signifies that even though a Guru provides the same spiritual 'polishing' to all, the outcome depends on the disciple's inner nature. A true disciple possessing virtues like faith and patience—the qualities of a gem—will become spiritually brilliant, whereas someone lacking these qualities will not attain the same luster.