A seed crystal is a small piece of single crystal material from which a large crystal of, usually, the same material is to be grown. The large crystal can be grown by dipping the seed into a supersaturated solution, into molten material that is then cooled, or by growth on the seed face by passing vapour of the material to be grown over it.
Examples where the a seed crystal is used to grow large boules or ingots of a single crystal include the semiconductor industry where methods such as the Czochralski process or Bridgeman technique are employed.