Quartz is the most common mineral in the Earth's crust. Well-formed crystals occur in clusters, geodes and veins. On the Mohs hardness scale of one (softest) to ten (hardest), quartz ranks a seven, meaning that it is quite hard. While many varieties of quartz exist, the most well-known is called "Rock Crystal" which is often colourless with a glassy lustre. Quartz crystals have no cleavage; therefore, the crystals do not break along the crystal face. If broken, quartz exhibits a conchoidal fracture. Because of the fracture, quartz crystals will break into sharp pieces.
Don your safety gear. At a minimum, you should wear safety glasses and a particulate mask. Gloves and a full mask can protect your skin from cuts.
Wrap the quartz crystals in a towel. Use old towels because they will likely tear because of the sharp crystal edges.
Place the wrapped crystals on a hard surface such as a concrete sidewalk or patio. Choose a spot that can withstand a strong force from a hammer and where you are not concerned with scratching the surface.
Hit the crystals with the rock hammer. To create the small shard size, you will likely need to strike the crystals repeatedly. If necessary, unwrap the crystals after your initial break and rewrap the large pieces to repeat the process. The natural fracture of the rock creates smaller shards of quartz as you continue to break the crystals. The benefit of using the rock hammer is that you can control the force that you exert on the crystals, thus controlling the size of the shards.
Use care when handling the quartz crystal shards; they may be very sharp.