Everything around us is made up of tiny building blocks called atoms. Different types of atoms combine in many different ways to make every material in the universe. A crystal is a material where the atoms are arranged in an orderly pattern. You can think of the atoms in a crystal as people sitting in a movie theater: everyone has an assigned seat, and no one is too close or too far from anyone else.

For a crystal to form, every atom needs to know where to go to form the pattern. The way this happens is a process called nucleation and growth. First, a small number of atoms come together, forming a tiny ordered crystal. This is called the nucleation step. Then, more atoms are added to the tiny crystal, making it grow. This is called the growth step. During growth, each atom added to the crystal has a specific place it can go, just like taking the open seat next to your friend at a movie theater—you only have one option of where to sit. In this way, the pattern of atoms created during the nucleation step is copied to every atom in the growth step, and an ordered crystal forms.

In general, light and darkness do not affect the growth step of crystals. The atoms that are added to the growing crystal still have only one allowable place to go. However, light can affect the nucleation step. Sometimes, atoms can only come together to form a small crystal when the conditions are right. For example, a crystal might not form on a perfectly smooth glass surface, but might form on a rough glass surface. And, some surfaces experience a chemical reaction when light hits them. These chemical reactions can make the surfaces more likely to grow crystals. So, there are some crystals that take a long time to form on a surface normally, but nucleate much more quickly when that surface is exposed to certain kinds of light. However, not all light is the same. Visible light that you and I can see isn’t very likely to change the chemistry of surfaces. However, higher energy light, like ultraviolet light or even X-rays, is much more likely to cause a chemical reaction and impact crystal nucleation!