Could you explain the analogy of the stone and the gem that Sai Baba uses when talking about a Guru's influence?

In Chapter 19, Sai Baba uses the analogy of a stone and a gem being polished on a whetstone to explain the effect of a Guru's guidance on different disciples. He points out that while both are subjected to the same process, the results differ vastly. A gem becomes a 'brilliant jewel' or a 'sparkling diamond,' while a stone merely becomes smooth, remaining a stone. This metaphor suggests that a Guru provides the opportunity for transformation to all, but the disciple's inherent quality and readiness determine the outcome. To truly gain the 'luster of a gem' and achieve spiritual brilliance, a disciple must possess the necessary inner qualities, like the faith and patience Sai Baba emphasizes.