Update [1/23]: Sold out at Amazon, but still at Walmart.

Amazon has the Pumie Pumice Stone Scouring Stick (HDW-12T) for $1.98 with free shipping on $25+ or with Prime (extra 5% off for 5+ S&s orders). Same price at Walmart.

Use to remove stubborn, deep stains on porcelain, ceramic tile, concrete, and iron.