Chazal (Sotah 36b) teach that when Yosef HaTzaddik almost fell prey to the blandishments of Potifar’s wife, an image of Yaakov Avinu, his father, appeared before him and said, “Yosef, your brothers’ names will eventually be engraved upon the stones of the Ephod, and your name (as of now) is destined to be included among them. Do you want your name to be omitted (if you sin)?” When Yosef heard this, he immediately withdrew. Potifar’s wife was not going to cause him to be deprived of his spiritual destiny.
So much for Yosef. What about Reuven and Yehudah? It is not as if they were not involved in an activity that reflected a degree of lapse in morality (relative to their elevated spiritual plateau); yet, their names were not omitted from the stones. Veritably, this is because of their teshuvah, repentance. Does this mean that had Yosef sinned and subsequently repented, his teshuvah would have been deemed unacceptable – and his name omitted from the Ephod? If so, why was he held to a different standard than Reuven and Yehudah? [Simply, we can say that whatever questionable moral decline they experienced, it was certainly not the same as an immoral relationship with the wife of his master. True, he did not initiate the sin, in that he was seduced and compelled by an unsavory woman bent on committing an iniquitous act of infidelity. Nonetheless, the egregiousness of the sin should have sufficiently distanced him from its perpetration.]
Horav Shimon Schwab, zl, offers a compelling insight into the Shoham stone’s function and Yosef’s connection to it. The Kohen Gadol’s Choshen, Breastplate, contained twelve precious stones upon which the names of the twelve tribes were engraved. The stone on the Breastplate which corresponded with Yosef’s name was the Shoham (onyx) stone. The Shoham was unique in the sense that, in addition to its placement upon the Ephod, two additional Shoham stones were on the shoulder straps of the Ephod. Six names of Tribes were engraved upon each of these stones, so that all twelve names were on both stones. In other words, Yosef’s name was engraved twice upon the Shoham stones – once on the Ephod and once on the kispos ha’Ephod. Yosef’s (Shoham) stone was the foundation stone upon which the names of all the tribes was engraved. This is why Yosef is called tzaddik yesod olam, ‘the tzaddik, righteous person, foundation stone of the world.”
Had Yosef fallen prey to Potifar’s wife’s seductive efforts and subsequently repented, his name would (like Reuven’s and Yehudah’s) still be etched on the Ephod. He would, however, have forever lost his tzaddik yesod olam status, because he had blemished it with sin. He no longer would have been the untainted, quintessential tzaddik. He was the tzaddik who had erred, repented and now was once again a tzaddik. When one is a tzaddik yesod olam, he has no room for error. When Yaakov Avinu appeared to Yosef to inform him that if he sinned his name would not appear on the Ephod, he meant that, while his name might have a place on the Choshen, his designated stone, the Shoham, would no longer serve as the foundation stone for all Klal Yisrael.