Can you explain the analogy of the gem and the stone from Chapter 19 and what it reveals about the Guru-disciple relationship?

The analogy of the gem and the stone, found in Chapter 19, serves to illustrate how a disciple's inner nature affects their ability to receive a Guru's teachings. In the metaphor, both a gem and a stone are subjected to the same process on a whetstone, which represents the Guru's guidance. While the process is identical for both, the outcome is vastly different: 'the gem becomes a brilliant jewel, while a stone remains smooth by its own nature.' This reveals that a Guru's grace requires a receptive vessel. A disciple with the inherent qualities of faith and patience (the gem) will be transformed, while one lacking these qualities (the stone) will only receive a superficial effect and cannot attain true brilliance.