Black stone
The prophet sallallaahu alaihi waslaam has said: “The Black Stone had been descended from the Paradise. That time it was whiter than the milk. The sins of people have turned this to be black. (Ahmed,Tirmizi)
The Mufti Ahmed Yar Khan Naeemi Rahmatullah alaih writes:  Hajre aswad (The Black stone) was like a crystal clear glass or a blotting paper.  As the crystal clear glass getting accumulated by the dirt and soil and also absorbs the ink from the wet letters written on another paper, the Hajr Aswad (Black stone) too has been turning black due to touching by us the sinners or by constant touching of the past idolaters. The respected Sufis used to say when the Hajre aswad (Black stone) has become black due to our sins, our hearts too get rust by our sins like the good ones too become bad by the friendship of bad people. Hence, it is better to avoid the bad people. Rab ta’la says:“فَلَا تَقْعُدْ بَعْدَ الذِّکْرٰی مَعَ الْقَوْمِ الظّٰلِمِینَ“ The color changes due to the effect of the comradeship.