I am glad you asked this question, since just today I attended a talk about a crystal structure of a very important bacterial protein.

One very important use of crystals in science is to solve protein structures. Humans alone express tons of different proteins that are essential for life. Other organisms, like bacteria, express several proteins as well, many of which are involved in pathogenesis, that is, the bacteria s ability to infect us and cause disease.

Protein structures are very useful in biochemistry and can lead to the development of drugs to inhibit or enhance enzymes, proteins that are involved in catalyzing reactions in the cell.

For instance, knowing the protein structure of a bacterial enzyme involved in pathogenesis can be useful for designing potent drugs to inhibit its actions when bacteria have invaded our cells. This is just one example of the importance of protein structures in science.

You may wonder how scientists grow their protein crystals and how they determine the protein structure from the crystal. I have never grown protein crystals but I know several people who have and it is not an easy process and requires a particular skill (and a lot of luck) to grow protein crystals that will be useful for obtaining data.

Crystallographers (scientists who grow crystals) use high concentrations of protein in a solution with various salt concentrations and different methods of growth to stimulate crystal formation. The protein crystal is then hit with X-ray beams and the intensities and directions of the scattered light of the crystal are determined. Through lots of data collection and analysis, the 3D protein structure is determined. I am sure you have seen pictures of protein structures maybe in your textbook or online. It is a wonderful technology to be able to generate a 3D protein structure from a small crystal! And very useful for biochemists who are trying to figure out how enzymes work.

Crystals actually have a huge number of uses in science, and whatever I will list here since it comes to my mind will only represent a small fraction of the actual uses.

Crystals often are useful because they have a very regular internal structure, and often have anisotropic properties,i.e. a certain property is different in different directions. That could be conductivity for heat, or optical properties, for example. I believe that your question is aimed mainly towards so called single crystals, where the whole piece of materials is one big crystal, as opposed to a conglomerate of many smaller crystals.

The graphite in your pencil is a crystal, but not a single crystal. Diamonds in jewelry, on the other hand, are the same chemical element (carbon), but in a different crystal structure and are single crystals.

The biggest (I would think) use of single crystals in science and anywhere are the huge, extremely pure single crystals of silicon that you never even see - they get sliced up as soon as they are grown and then are the basis for all microelectronics, such as computer chips. In science, some optical equipment is made from crystals. These can be special items, such as polarizers or frequency doublers (the latter turn e.g. red laser light into (less intense) green laser light) or lenses and prisms made from e.g. table salt for light that is invisible to the eye and blocked by glass.

Other crystals are used to select specific wavelengths or colors not only from white light, but from similarly "white" X-ray beams.

As I said, there are many other applications of crystals that I must be forgetting at this moment. The unfortunate thing for those of us who admire the beauty of crystals (and who don't) is that as with the silicon crystals, most of the time the tools that are made from the crystals don't show them anymore... So to see them, you have to make them - and many scientists make crystals on almost a daily basis, be it inorganic crystals as new materials or catalysts, or crystals of biological proteins, and analyze their structure. And most likely they do that analysis using X-ray beams that make use of well-known crystals!

In the context of biological sciences, growing crystals of proteins is a large field that is of huge interest not so much because the crystals will be used for applications, but they are used to determine the structure of the proteins, which gives us insight into how they are built and how they work. And since proteins perform just about every function in our body on a molecular level, this is extremely interesting information.

Crystals are very important in science for various reasons. One of the major reasons is that they give us information about the structure of compounds.

In biology crystals of proteins and larger compounds also give an idea of what they are made of which help scientists further understand the functions of the molecules.

Comparison of crystal structures may also play a role in predicting reactivity or other properties of compounds.

It is also important to be able to compare properties of a crystalline material to that of the amorphous solid. For example, CDs and DVDs are made up of materials that change from amorphous solids to crystalline solids and vice versa when hit with a certain light...this enables us to write and store information on them.