Baba tells a story comparing a gem and a stone being polished. What was he trying to teach with this analogy?

The analogy of the gem and the stone, found in Chapter 19, is used by Sai Baba to explain the role of a disciple's inherent qualities in their spiritual development. He states that a powerful Guru still expects the disciple to possess "strong faith at the Guru’s feet and the strength of courage which is patience." He then compares this to a gem and a stone being rubbed on the same whetstone. Both undergo the same polishing process, but only the gem becomes a brilliant jewel, while the stone just becomes smooth. This illustrates that while a Guru's grace is available to all, only a disciple with the intrinsic qualities of faith and patience (the gem) can be fully transformed by it.