Hajr-e-Aswad, also known as the Black Stone, has an interesting history. According to Islamic tradition, it was brought down from heaven and given to Prophet Ibrahim (AS) to place in the corner of the Kaaba. Initially, it was milky white, but it turned black due to the sins of humanity.
Hajr-e-Aswad was once a complete single stone, but it is now in pieces of eight that have been affixed to a large silver frame. Six of these pieces were taken to Istanbul, Turkey, and are displayed in various mosques and shrines.
In January 930, the Qarmatians attacked Makkah during Hajj, killed around 30,000 pilgrims, and looted the Kaaba, including Hajr-e-Aswad. They held it for 23 years, and it was recovered only after the Abbasids paid a huge ransom.
According to the Prophet Muhammad (PBUH), on the Day of Resurrection, the Black Stone will testify to whoever touched it in truth. Additionally, it is one of the eight places in the Kaaba where prayers are accepted.