Once when the chassid Reb Monya Monoson had a private audience with the Rebbe Rashab, their discussion concerned several simple individuals whom the Rebbe praised. This surprised Reb Monya and he asked the Rebbe, "What's the big deal [about them]?" The Rebbe responded, "They possess special qualities" to which Reb Monya replied, "I don't see it."

As was known, Reb Monya was a successful diamond merchant. When the Rebbe asked him whether he had a pouch of diamonds with him, he responded affirmatively. "However," he said, "due to the lack of sunshine, it would not be advisable to examine them presently." Later, Reb Monya took the pouch into a second room and displayed the diamonds, pointing out the incredible quality of one specific stone. The Rebbe remarked, "I don't see anything special in it," to which Reb Monya replied, "[For that,] one must be a maven (expert)."

The Rebbe then told him, "When it comes to seeing the special qualities of a Jew's soul, one also has to be a maven."

Sefer HaSichos 5705, p. 41