What is the ruling on placing the Black Stone on henna and nail polish?

If the purpose of the stone is to intensify the blackness of henna, then there is no harm in it, nor in using black nail polish, as long as its harm is not proven. If this polish has a physical bulk that prevents water from reaching the nails, then ablution is not valid with its presence. But if it is merely a color, like henna, which does not prevent water from reaching, then there is no harm in performing ablution while it remains.