In most South Indian Temples Idols Are Anointed With Oil And Lot Of Liquids Are Poured As Offerings. Black Stones Remain Unaffected By The Chemicals In Various Liquids Poured On It.
The Decoration of The Idol With the Holy Ash And Sandal Paste Is A Common Practice. The Black Stone Enhances the Beauty Of the Decoration By Contrast.
Also The South Indian Idols Are Made As Per The Agama Shashtra Which Stipulates That Idols Be Made From Black stone.