The Black Stone is the right hand of Allah with which He shook hands with His creation

It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "On the Day of Resurrection, the Black Stone will come bigger than Mount Abu Qubays. It will have a tongue and two lips, and it will speak about the one who touched it with a sincere intention. It is the (figurative) right hand of Allah with which He shakes hands with His creation."
A detailed supporting narration (shahid) of this hadith has also been reported, but it does not meet the condition of the two Shaykhs (Bukhari and Muslim), because they did not use Abu Harun Umarah bin Juwayn al-Abdi as evidence:

It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him): We performed Hajj with Sayyiduna Umar bin al-Khattab (may Allah be pleased with him). When he entered the tawaf, he stood facing the Black Stone and said: "I know well that you are merely a stone that can neither harm nor benefit, and had I not seen the Messenger of Allah (peace and blessings be upon him) kiss you, I would never have kissed you." Then he kissed it. Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said to him: "Rather, O Commander of the Believers, it does indeed harm and benefit." He asked: "On what basis do you say this?" He replied: "On the basis of the Book of Allah, the Mighty and Majestic." He asked: "Where is this stated in the Book of Allah?" Sayyiduna Ali (may Allah be pleased with him) said that Allah Almighty says: "And when your Lord took from the children of Adam, from their backs, their descendants, and made them testify about themselves: Am I not your Lord? They all said: Yes indeed! (we testify)." [Surah al-A'raf: 172] Allah created Adam (peace be upon him) and passed His hand over his back, and made them affirm that He is the Lord and that they are all His servants. Then He took their covenants and pledges from them and wrote them in a scroll. At that time this stone had two eyes and a tongue, and Allah said to it: Open your mouth. It opened its mouth, and Allah placed that scroll in its mouth and said: On the Day of Resurrection, bear witness to the faithfulness of whoever comes to you. And I bear witness that I heard the Messenger of Allah (peace and blessings be upon him) say: "On the Day of Resurrection the Black Stone will be brought, and it will have a most eloquent tongue that will testify to tawhid in favour of the one who touched it." So, O Commander of the Believers, it does indeed benefit and harm. Then Sayyiduna Umar (may Allah be pleased with him) said: "I seek refuge with Allah from living among a people in which you are not present, O Abu al-Hasan (Ali)."