Serpentinites are formed by the weathering and breakdown of ultramafic rocks. This process may have contributed to the start of life on earth so it is pretty fascinating stuff.
Serpentinite is often found in ophiolite complexes. Ophiolite complexes are regions where oceanic crust, which is generally subducted and recycled into the earth, is instead emplaced onto continental crust. One famous ophiolite complex is the Lizard in Cornwall. There are also a number of ophiolite sites in Oman which offers important clues to the formation of the Himalayas.
Some serpentinites are also a precursor to the two officially recognised types of jade; nephrite and jadeite. They form by metasomatic processes and involve the movement of hot fluids through serpentinised rocks.