What is the meaning behind Baba's analogy of the stone and the gem being polished on a whetstone?

In Chapter 19, Sai Baba uses the analogy of a stone and a gem to explain the role of a disciple's innate qualities in the process of spiritual growth. He points out that while both a stone and a gem can be polished on the same whetstone, the stone only becomes smooth, whereas the gem becomes a brilliant jewel. This metaphor illustrates that a Guru's guidance, like the whetstone, can only work upon the material it is given. Baba clarifies that a Guru expects 'the disciple's wisdom; strong faith at the Guru’s feet and the strength of courage which is patience.' A disciple with these qualities is like the gem, possessing the potential to become brilliant. A person lacking them is like the stone; they may be improved by the Guru's presence but cannot achieve the same spiritual luster.