My question is, where did that Black stone come from? Who put it there and what was the purpose?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The Black Stone descended from Paradise along with Aadam . At first it was white, then it became black due to the sins of the children of Aadam .

Ibn ‘Abbaas narrated that the Prophet said: “The Black Stone descended from Paradise while being whiter than milk, then the sins of the children of Aadam turned it black.” [At-Tirmithi – Al-Albaani classified it as Saheeh (sound)]

Another narration reported by Al-Azraqi reads: “…The Black Stone descended glittering because of its extreme whiteness and then Aadam hugged it because he felt so happy to see it.” Moreover, Abdullaah ibn ‘Amr narrated, “Jibreel (Gabriel) descended with the Black Stone from Paradise and put it where you know, and you are upon much good as long as it [Black Stone] is with you, so hold fast to it as much as you can, as it is about to be taken back to where it came from.”

Perhaps the reason why the Black Stone was sent down (to the earth) from Paradise is what some scholars mentioned that when Aadam was sent down from Paradise, he felt very lonely and he yearned very much for it [Paradise], and he became extremely attached to it. Thus, Allaah The Almighty sent down the Black Stone from Paradise so that Aadam would be comforted and feel at ease with it and to alleviate his longing for it [Paradise]. Allaah The Almighty made kissing it and touching the Black Stone a means for the forgiveness of one’s sins, as the Prophet said: “Indeed, Allaah will resurrect the Black Stone on the Day of Judgment with two eyes by which it sees and with a tongue by which it speaks and it will bear witness for whoever really touched it.” [At-Tirmithi and Ibn Khuzaymah]

The Prophet also said: “Touching the Black Stone and the Yemini Corner completely wipes out the sins.” [At-Tirmithi, Ibn Hibbaan, Al-Haakim and Ath-Thahabi]

Allaah Knows best.