Mineral Mystery EarthCache

Rocks are grouped into three main types: igneous, sedimentary, and metamorphic based on how they form.

• Igneous rocks form from cooled lava or magma.
• Sedimentary rocks form from layers of sediments or chemical deposits.
• Metamorphic rocks form when existing rocks change under heat and pressure.

By observing a rock’s texture, color, layering, and hardness, we can identify which type it is and learn how it formed. The dark, fine-grained rock you see here offers clues to its geologic origin.

At first glance, this dark boulder might seem ordinary, but a closer look at its color, texture, and markings can help us uncover its story. Geologists use these clues to identify rock types and learn how they formed.

What Kind of Rock Is This?

Option 1: Chert (Sedimentary)

• Color: Usually gray, black, or brown; this one’s dark gray to black with rusty streaks.
• Texture: Very smooth and hard, with a slightly glassy look and small pits from weathering.
• Formation: Forms when tiny silica particles accumulate in marine sediments, often within limestone. Over time, they harden into a dense, durable rock.

Option 2: Basalt (Igneous)

• Color: Dark gray to black, often uniform.
• Texture: Fine-grained and smooth, sometimes with small holes from gas bubbles in cooling lava.
• Formation: Created when lava cools quickly at the Earth’s surface, usually from volcanic eruptions.

Option 3: Hornfels (Metamorphic)

• Color: Dark gray to black, sometimes with a dull sheen.
• Texture: Very hard and dense, often with a baked or “splintery” surface.
• Formation: Forms when existing rock is “baked” by nearby magma, changing its structure without melting.

Each of these rocks can appear similar at first: dark, fine-grained, and tough, but their origins are completely different. By examining color, hardness, and surface texture, you can begin to determine which story this rock tells.