  When a system undergoes a transition from a liquid to a solid phase, it
passes through multiple intermediate structures before reaching the final
state. However, our knowledge on the exact pathways of this process is limited,
mainly due to the difficulty of realizing direct observations. Here, we
experimentally study the evolution of symmetry and density for various
colloidal systems during liquid-to-solid phase transitions, and visualize
kinetic pathways with single-particle resolution. We observe the formation of
relatively-ordered precursor structures with different symmetries, which then
convert into metastable solids. During this conversion, two major
cross-symmetry pathways always occur, regardless of the final state and the
interaction potential. In addition, we find a broad decoupling of density
variation and symmetry development, and discover that nucleation rarely starts
from the densest regions. These findings hold for all our samples, suggesting
the possibility of finding a unified picture for the complex crystallization
kinetics in colloidal systems.
