To remove grease, dirt, and soot build up from the bricks or stones in or around your fireplace, you can use this simple homemade solution: Make a mixture of 1 cup laundry detergent flakes and 4 cups hot water, then add a half pound of powdered pumice (available at hardware stores like Home Depot). To this mixture, add 1/2 cup of ammonia. Now you are ready to clean. Use a brush with stiff bristles to apply the mixture to the dirty bricks and scrub, rinse, and repeat, until the area is clean.
Source: Home Improvement Tips