Can you explain the analogy of the gem and the stone being polished, and what it implies about the Guru-disciple relationship?

The analogy presented in chapter 19 compares a gem and a stone both being rubbed on a whetstone. While they undergo the same polishing process, the results differ vastly: the gem becomes a 'brilliant jewel,' while the stone merely becomes smooth. In the context of the Guru-disciple relationship, the whetstone represents the Guru's teachings and presence. The analogy implies that while the Guru's guidance is available to all who approach him, the spiritual transformation of the disciple depends on their own inherent quality and readiness. A disciple with the innate potential and the necessary faith and patience (like a gem) will gain immense spiritual luster, whereas another (like a stone) may only experience superficial changes without a fundamental transformation.