Give the fireplace a base coat of white paint. Whatever materials you've used to build the fireplace will have their own color and texture with their own inconsistencies. For the best imitation stone effects, you will want to be working with a neutral base.
Sketch the outline of the stone shapes. Outline either irregular natural stone shapes using roughly round and oval drawings in different sizes and positions, or draw staggered brickwork patterns if you want a fireplace made of carved stone bricks. For the latter, achieve consistency by cutting out a rectangular cardboard pattern to trace for every brick.
Paint the mortar first. Coat the spaces between the stone outlines with off-white or gray paint and let dry. Create the impression of pores in the mortar by spatter-painting over this first coat in a slightly darker shade of the same color paint; apply paint to an old toothbrush and run your finger through the bristles to cast off speckles of paint. Let this dry as well.
Add the stone detail. Fill the stone outlines with a solid coat of light earth-tone paint colors (beige, off-white or gray), then add a partial, splotchy second layer of a different color using a sponge. You can also use a granite-effect spray by covering the "mortar" sections with masking tape, spraying, then removing the tape.
Create shadows around the bricks. Paint dark outlines around the sides and bottoms of the stones. Make the bottom lines slightly thicker. This will give the illusion of dimension.