Birth is a tough process, particularly for crystals. Before a crystalline structure forms, atoms or molecules mingle in a solution, making and breaking attachments. After enough particles gather in a cluster, and surpass a critical size, crystal growth speeds along.
The initial clumping, called nucleation, helps determine a crystal's size, composition, and other properties. Scientists have developed theories about nucleation for decades, but they hadn't actually witnessed nuclei forming in solution.