The Hajar al-Aswad (the Black Stone) is not your average rock, it has a weird history. The animist Arabs worshipped that meteorite long before Islam. Mohammed simply took possession it, kissed it, and it's still being effectively worshiped in the annual Islamic rituals.
Now days it's set in gold and hangs on a wall of the Holy Kaaba. Muslims making hajj are supposed to try and kiss it like Mohammed allegedly did, or at the very least wave at it as they do the ritual 7 circles around the Kaaba. (I swear I'm not making any of this up, you can research it for yourself.)
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According to Islamic lore the meteorite dropped at Adam's feet after he was expelled from Eden. It was white originally, but turned black from absorbing the sins of men.
BTW, if you ever wondered what else goes on during the hajj after they try to kiss that filthy rock, it gets even more bizarre. The faithful get to run seven times between Mt. Safa and Mt. Marwah (which aren't mountains, just rises in the desert), and then they proceed to Mina, a few miles from Mecca. That's where they do "The Throw". They throw pebbles over their shoulders at 3 stone pillars that represent devils.
You can imagine how many accidents and deaths occur annually to Muslim pilgrims during all this, particularly because the Saudi hosts aren't exactly famous for their organizational abilities.:)