Chert is a hard, sedimentary rock made primarily of microcrystalline quartz. It often appears in shades of gray, brown, or green and can be found in various geological settings, including riverbeds and ocean floors. Chert forms from the accumulation of tiny silica particles, which can come from the remains of marine organisms or from the chemical precipitation of silica in water. This rock has been used by humans for thousands of years, especially by prehistoric cultures for making tools and weapons. Its sharp edges make it ideal for crafting items like arrowheads and knives. Today, chert is also studied by geologists to understand ancient environments and the Earth's history, linking it to topics like sedimentary rocks and fossils.