Can you explain the analogy of the gem and the stone that Baba uses to describe the Guru's influence on different disciples?

In Chapter 19, Baba employs the analogy of a gem and a stone being polished on the same whetstone to illustrate how a Guru's guidance affects disciples based on their inherent nature. Both the stone and the gem undergo the identical process of rubbing. However, the outcome is vastly different: the stone merely becomes smooth, while the gem is transformed into a brilliant, sparkling jewel. This powerful analogy suggests that while a Guru provides the same teachings and opportunities for growth to all, the result depends on the disciple's own readiness and capacity. A disciple with the inherent potential of a gem will gain immense spiritual luster, whereas another, like a stone, will see some surface-level improvement but will not undergo the same profound transformation.