6. Reading the Rocks: Identifying & Using Them

Geologists are detectives: a rock's texture tells the story of how it formed, and that lets you identify it and even put a number on Earth's past.

A simple identification key:

• Interlocking crystals, no layers or bands → igneous (big crystals = slow/intrusive; tiny = fast/extrusive).
• Flat layers or visible grains, maybe fossils → sedimentary.
• Bands of aligned minerals, or a shiny, recrystallized look → metamorphic.

Rocks and minerals are also everywhere in daily life: granite countertops, marble statues, limestone in cement, coal and oil for energy, salt for food, and metals refined from ore minerals. Reading rock layers also lets geologists date events: by superposition, lower layers are older, so a fossil in a deep layer is older than one above it.

Reading rock data (a GED® skill). GED® items may give a property table (hardness, luster, streak) or an identification key and ask you to classify a sample. Match the clues to the key, and use only what the data shows — for instance, a sample with clear mineral bands is metamorphic.

⚠️ Common misconception: you can't reliably name a rock by color alone. Use texture — crystals, layers, or bands — which records how it actually formed.

💡 Tip: identify by texture: crystals = igneous, layers = sedimentary, bands = metamorphic.

📚 Sources: USGS Rocks and Minerals; National Park Service Geology; Tarbuck & Lutgens, Earth Science; Marshak, Essentials of Geology; Mohs (1812).