What was Sai Baba trying to explain with his analogy about a stone and a gem being polished on a whetstone?

Sai Baba used the analogy of the stone and the gem in Chapter 19 to illustrate the importance of a disciple's inherent qualities and readiness. He explained that when both a stone and a gem are rubbed on a whetstone, they undergo the same process. However, the outcome is vastly different: the gem becomes a brilliant, sparkling jewel, while the stone merely becomes smooth. This metaphor highlights that even with the guidance of a powerful Guru (the whetstone), the disciple's receptivity and inner nature determine the result. A Guru, he states, expects the disciple to possess 'strong faith at the Guru’s feet and the strength of courage which is patience,' which are the qualities that allow them to be polished into a jewel.