Pumice Stone Fungus????
I did not know there was such a thing until recently I picked up my pumice stone that I keep in the shower and it was covered with a black, blotchy, mold like fungus. I also did not know you had to properly clean, care for and store them. So I googled "Pumice Stone Fungus" and here is what I found.
How to Care for Pumice Stones
Wet pumice stones before every use. Using a dry pumice stone can damage your skin and increase breakage of the stone. Always check your stone for jagged edges or cracks before using it to remove corns or calluses.
Rinse often during use to remove dead skin cell flakes and prevent dulling of the stone. Hold the stone under warm, running water until all loose particles wash away.
Clean pumice stones after every use to remove dead skin and other debris. Wet the stone and a toothbrush with warm water. Apply liquid dish detergent to the toothbrush, and then use the brush to scrub all sides of the stone for several minutes. Rinse the stone well in warm water to remove soap, dirt and oil.
Sterilize pumice stones to eliminate bacteria and fungus and prevent skin infections during use. Boil the stone in a small pot of water for five minutes. You can also soak the stone in a solution of four cups of water and three tablespoons of bleach for three to five minutes. Rinse the stone very well before drying or using.
Allow pumice stones to air dry completely before storing. Storing your stone while wet will encourage the growth of fungus. Place the stone on a small, clean towel after washing to encourage air circulation around the stone. Alternatively, you can place the stone outside in the sun to dry.