black granite fountain

Aloha,

 

I have recently taken on the job of cleaning the Holy Water Fountain at our church.  All I know is that the fountain is black and appears to be of marble or granite.  My challenge is that it has two tiers, and there are calcium/mineral stains where water from the upper tier flows onto a shelf and then down into the lower tier.  I was told not to put any chemicals into the fountain, so this limits me in preventing the stains.  However, I am wondering if there is something that I can use externally to remove the deposits, and hinder further growth/buildup?  Your advice will be greatly appreciated.

 

Mahalo (thank you),

Beverly