The heart principle.
Carving crystals into a shap help to enhance specific properties of the stone and magnify its use in your life. Shapes don't just make crystals pretty it enables you to work with the stone in different ways and with different purpose. Everyone has heard of sacred geomtery by now and its use to direct energy. As above so below, different shapes help direct the energy of the crystal in different ways.
The heart represents love and connection. As such, crystal Hearts help you to connect to the heart principle of our Heart Chakra. You may place them over your heart during meditation, times of sorrow, as you relax, chant etc. They also make great pocket stones to carry with you and hold throughout the day as you need them.