This is actually simple. One by one is how crystals grow. You don't cool down liquid silica and it suddenly becomes crystal. Instead it has a front where silica

This is actually simple. One by one is how crystals grow. You don't cool down liquid silica and it suddenly becomes crystal. Instead it has a front where silica atoms find a good spot which creates more spots. Growing amorphous substance would happen largely the same way. The requirements for the mirror and the behaviour of glass are much more exciting to me.