MA key ingredient is cocoa butter. It is a fat and it can come in at least six different crystalline forms. This means that the atoms are the same but they are arranged differently. The different arrangements can lead to different properties including melting point, how easily it snaps, strength, glossiness and texture. It’s a bit like lego bricks. You can use the same bricks to make different structures; some are stronger, and some look better.
The ability of the structure to take on many different crystalline forms is called polymorphism. ('poly' means many; 'morph' means shape). The details of the polymorphism of chocolate are very complex and this is still an area of active research. One of the six polymorphs – form V – has a far superior taste and texture than the others. It is also the glossiest and snaps well.