This is a solid made of atoms or molecules arranged in a 3-D pattern. Materials of this sort are called “crystalline.” If you could zoom way in to look inside a crystalline material, you’d see a repeating building block. This building block is called a “unit cell.” In the common crystal table salt, or sodium chloride, the sodium and chlorine atoms arrange in a cube-shaped unit cell. But unit cells come in other shapes too. Some are hexagonal prisms. Others are more like a cube that’s had its top pushed over.
One way scientists figure out the positions of atoms or molecules in a crystal is by blasting them with X-rays. The way X-rays scatter when colliding with a material can reveal how the parts are stacked up.
The word “crystal” brings gems to mind, for good reason. A lot of gemstones are crystals. For example, a diamond is a crystal made of carbon atoms in a repeating pattern. Some people attribute special healing powers to crystals. But there’s no evidence that those expensive rocks can protect someone any more than the salt in the shaker on the counter.