Can you explain the analogy of the stone and the gem in relation to a disciple's development?

Chapter 19 uses the analogy of a stone and a gem to explain how a Guru's guidance affects different disciples. Both the stone and the gem are polished on the same whetstone, representing the Guru's teachings. However, their intrinsic natures lead to different outcomes: the stone only becomes smooth, while the gem transforms into a brilliant jewel. This illustrates that while the spiritual process is the same for all, the disciple's inner qualities are what determine the result. A disciple who possesses inherent faith and patience (the gem) will become spiritually radiant, while one who lacks these qualities (the stone) will only experience a superficial change. The Guru's process reveals and perfects the disciple's true nature.