Deep beneath the geysers and hot springs  of Yellowstone Caldera lies a magma chamber produced by a  hot spot in the earth’s mantle. As the magma moves towards  the Earth’s surface, it crystallizes to form young,  hot igneous rocks. The heat from these rocks drives  groundwater towards the surface. As the water cools, ions precipitate out  as mineral crystals, including quartz crystals from silicon  and oxygen, feldspar from potassium, aluminum,  silicon, and oxygen, galena from lead and sulfur. Many of these crystals have signature  shapes— take this cascade of pointed quartz,  or this pile of galena cubes. But what causes them to grow into these shapes again and again? Part of the answer lies in their atoms. Every crystal’s atoms are arranged  in a highly organized, repeating pattern. This pattern is the defining  feature of a crystal, and isn’t restricted to minerals— sand, ice, sugar, chocolate, ceramics,  metals, DNA, and even some liquids have  crystalline structures. Each crystalline material’s atomic  arrangement falls into one of six different families: cubic, tetragonal, orthorhombic,  monoclinic, triclinic, and hexagonal. Given the appropriate conditions, crystals will grow into geometric shapes that reflect the arrangement  of their atoms. Take galena, which has a cubic structure  composed of lead and sulfur atoms. The relatively large lead atoms are arranged in a three-dimensional  grid 90 degrees from one another, while the relatively small sulfur atoms  fit neatly between them. As the crystal grows, locations like these attract sulfur atoms, while lead will tend to  bond to these places. Eventually, they will complete the grid  of bonded atoms. This means the 90 degree grid pattern  of galena’s crystalline structure is reflected in the visible  shape of the crystal. Quartz, meanwhile, has a hexagonal  crystalline structure. This means that on one plane its atoms  are arranged in hexagons. In three dimensions, these hexagons are  composed of many interlocking pyramids made up of one silicon atom  and four oxygen atoms. So the signature shape of a quartz  crystal is a six-sided column with pointed tips. Depending on environmental conditions, most crystals have the potential to form multiple geometric shapes. For example, diamonds, which form deep in the Earth’s mantle, have a cubic crystalline structure and can grow into either cubes or octahedrons. Which shape a particular  diamond grows into depends on the conditions where it grows, including pressure, temperature,  and chemical environment. While we can’t directly observe growth  conditions in the mantle, laboratory experiments have shown some evidence that diamonds tend to grow into cubes at  lower temperatures and octahedrons at higher temperatures. Trace amounts of water, silicon,  germanium, or magnesium might also influence a diamond’s shape. And diamonds never naturally grow into the shapes found in jewelry— those diamonds have been cut to  showcase sparkle and clarity. Environmental conditions can also  influence whether crystals form at all. Glass is made of melted quartz sand, but it isn’t crystalline. That’s because glass cools  relatively quickly, and the atoms do not have time to arrange themselves into the ordered structure  of a quartz crystal. Instead, the random arrangement  of the atoms in the melted glass is locked in upon cooling. Many crystals don’t form geometric shapes because they grow in extremely close  quarters with other crystals. Rocks like granite are full of crystals, but none have recognizable shapes. As magma cools and solidifies, many minerals within it crystallize at the same time and quickly run out of space. And certain crystals, like turquoise, don’t grow into any discernible geometric shape in most environmental conditions, even given adequate space. Every crystal’s atomic structure has  unique properties, and while these properties may not have any bearing on human emotional needs, they do have powerful applications  in materials science and medicine.