one of cocoa butter's six crystal forms has the properties we want.
Crystals are repeating patterns of atomic links. Depending on the angles between these links, the crystals have different properties. Here's a pair of examples:
Tempering chocolate involves heating it enough to break the bonds, then cooling rapidly to form the beta crystals that have the desired texture. The method I use suggests adding some unmelted chocolate during the cooling process, which partly cools the mixture further, but it may also provide a starting point for the crystals. Existing structures play a big part in how a crystal grows.
In the video below, the presenter drops a salt crystal into a supersaturated solution. The presence of the crystal causes salt in the solution to precipitate out and join the structure: