Can you explain the analogy of the stone and the gem that Baba used when talking about a Guru's expectations?

The analogy of the stone and the gem, which Sai Baba shares in Chapter 19, illustrates the importance of a disciple's inherent qualities in their spiritual development. Baba explains that both a common stone and a precious gem can be rubbed on the same whetstone. While the process is identical, the outcome differs: the gem becomes a 'brilliant jewel,' whereas the stone only becomes smooth. This metaphor signifies that a Guru provides guidance to all, but the disciple's capacity to receive it, rooted in their faith and patience, determines the result. A Guru expects 'strong faith at the Guru’s feet and the strength of courage which is patience' to truly transform a disciple into a spiritual 'jewel.'