Rabbi Ḥiyya bar Abba quoted Rabbi Yohanan as teaching that if someone claims to have already paid an obligation that was established by the courts – e.g. a man who divorces his wife and is obligated to pay her ketuba – he is not believed without proof. The Gemara offers a straightforward reason for this – an obligation that is based on a rule established by the courts is considered to have the strength and significance of an official document held by the plaintiff. Thus, without proof or witnesses, the defendant will not be believed on the strength of his claim.
Upon hearing this teaching, Rabbi Ḥiyya bar Abba pointed out to Rabbi Yohanan that his teaching was the underlying basis of a Mishna in Massekhet Ketubot (88b) which teaches that a woman who produces a geṭ, even without a ketuba, will still be able to collect what is owed her. Rabbi Yohanan accepted Rabbi Ḥiyya bar Abba’s point, but told him that, “Had he not lifted that haspa (a pottery shard), he would not have discovered the jewel underneath it,” which is to say, it was only because of his teaching that Rabbi Ḥiyya realized the significance of the Mishna in Massekhet Ketubot.
The analogy of the clay shard and the jewel can be understood simply as the difference between the valueless covering and the valuable hidden object. Tosafot, however, point to a more exact meaning, quoting Rabbeinu Tam as explaining that on the ocean floor there are large stones that look like pottery shards, and jewels can be found underneath them. Some suggest that the word haspa is, in fact, the name of the sea shell in which pearls are found, and the idea conveyed by Rabbi Yohanan is that if someone does not pay close attention to the simple sea shell, he will not succeed in finding the pearl.