I read things about it and it say it was white at first and if you kiss it it absorb your sins and it's why the stone is black.

More about it here:

4. Ibn `Umar (may Allah be pleased with him) quoted the Prophet (peace and blessings be upon him) as saying: “Touching them both (the Black Stone and Ar-Rukn Al-Yamani) is an expiation for one’s sins.” (At-Tirmidhi, Sunan, hadith no. 959. This hadith is classified as hasan by At-Tirmidhi and as Sahih by Al-Hakim (1/664), and Adh-Dhahabi agreed with him.)
(The aforementioned hadiths were quoted from a fatwa by the prominent Saudi scholar Sheikh Muhammad Salih Al-Munajjid, www.islam-qa.com)

[link to wikiislam.net]

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