What is the meaning of the analogy of the stone and the gem being polished on a whetstone?

The analogy of the stone and the gem, as described in Chapter 19, illustrates the difference in outcome for disciples based on their inherent nature and readiness. The whetstone represents the Guru's teachings or the spiritual process, which is applied equally to all. Both the stone and the gem are subjected to the same polishing process. However, the gem becomes a "brilliant jewel" or a "sparkling diamond," while the stone merely becomes smooth, remaining a stone. This suggests that while the Guru's guidance is available to everyone, only the disciple who possesses the innate qualities of a 'gem'—such as deep faith and patience—can truly transform and attain spiritual brilliance. A disciple who is like a 'stone' may receive some superficial benefit but cannot achieve the same profound transformation.