What is the meaning behind the analogy of the gem and the stone that Sai Baba uses in Chapter 19?

In Chapter 19, Sai Baba uses an analogy to illustrate the different outcomes of spiritual guidance. He explains that when a stone and a gem are both rubbed on a whetstone, they undergo the same polishing process. However, the result is vastly different: 'the stone remains a stone, but the gem becomes a brilliant jewel.' The whetstone can be seen as the Guru or the spiritual path, which provides the same process and opportunity to all. The analogy suggests that the disciple's inherent nature and receptivity are crucial. A disciple with the innate quality of a 'gem' will become brilliant and transformed by the Guru's guidance, while another, like a 'stone,' may be smoothed by the process but will not attain the same spiritual luster.