Calcite, the most common form of natural calcium carbonate (CaCO3), a widely distributed mineral known for the beautiful development and great variety of its crystals.
A wavefront is a surface over which an optical wave has a constant phase. For example, a wavefront could be the surface over which the wave has a maximum (the crest of a water wave, for example) or a minimum (the trough of the same wave) value. The shape of a wavefront is usually determined by the geometry of the source.
In science, a wave is defined as a transfer of energy. Ocean waves are called mechanical waves because they travel through a medium. The medium in this case is water. The water doesn't actually travel with the wave, but only moves up and down. It's the energy that travels with the wave.