How does the analogy of the stone and the gem explain why different disciples might have varying spiritual progress under the same Guru?

As explained in Chapter 19, Sai Baba uses the analogy of a stone and a gem being rubbed on the same whetstone to illustrate differences in spiritual outcomes. The process, representing the Guru's teaching and guidance, is the same for both. However, the result differs based on the inherent nature of the material. The text notes, "A gem becomes a sparkling diamond, while a stone remains smooth by its own nature." This implies that while the Guru provides the same opportunity for polish and refinement to all, a disciple's innate qualities, readiness, and capacity—perhaps their inherent level of faith and patience—determine the extent of their transformation. A receptive disciple (the gem) can become a brilliant jewel, while another (the stone) may only receive a superficial benefit.