Trimming a rock down to size for a tumbling machine, breaking open a geode, breaking a rock into small parts for easier removal or searching for fossils within a sedimentary rock are some reasons for breaking a rock. Regardless of the reason, a common hammer and chisel will do the job; however, specially designed tools are available from multiple online retailers.
Wash the rock with warm, soapy water to remove any dirt or detritus. Scrub any crevices or cracks with an old toothbrush. While this step isn't necessary, it will make it much easier to see cracks form as the rock is broken into pieces.
Dry off the rock with an old towel.
If the rock is small enough, place it on a flat, sturdy surface.
Position the flat-tipped chisel on the rock where the crack is to form. Have a plan in mind before beginning work. Know where the rock should break.
Lightly tap the top of the chisel with the hammer to score the rock. Move the flat tip of the chisel along the desired break line, lightly tapping with the hammer as you go. Continue scoring the rock until you circle around completely.
Repeat Step 5, only this time tap a bit harder with the hammer. For instance, hold the hammer two or three inches above the chisel head and let gravity pull the hammer downward. Work your way around the rock, following your previously made score line.
Continue tapping harder with the hammer until a crack forms along the score line. At this point, place the chisel at either end of the crack and tap with the hammer to extend the crack.
Continue extending the crack all the way around the rock until it breaks completely along the score line. If the rock is not hollow, the center of the break may be uniform in shape.
Position the tip of the chisel at a 20-degree angle to the rock and strike with the hammer to chip away small shards. Using this method, you can shape certain types of stone into a desired shape.
Use a broom or hand-held brush and dustpan to sweep up any rock shards after breaking.
Practice on chucks of earth before attempting to break valuable geodes or sedimentary rocks containing fossils. Once the rock is broken, there is no way to put it back together. Rock shards can be very sharp.
Always wear safety goggles when breaking rocks. Shards of rock can become flying shrapnel after a particularly fierce strike of the hammer.
Wear gloves to prevent your hands from getting cut on sharp rock shards.