Rocks have properties we can observe and test. Hardness tells us whether a rock is easy or difficult to scratch. Texture tells us if a rock feels smooth, rough, grainy, or glassy. Color varies from white to black and every shade in between. Luster describes whether a rock looks shiny or dull. Some rocks are heavy for their size while others are surprisingly light. Scientists use these properties to identify and classify rocks.
Give children a collection of diverse rocks and a simple recording sheet. Have them observe and describe each rock's color, texture, hardness, and luster. Test hardness by scratching rocks with a fingernail, a penny, and a nail. Use hand lenses (magnifying glasses) to look closely at texture. Sort rocks by one property at a time to see how grouping by different properties yields different results.
When scientists find a rock, they do not just say "it is a rock." They describe it carefully using its properties -- the specific characteristics that make one rock different from another. Learning to observe these properties is like learning a language for talking about rocks.
Hardness is one of the most useful properties. It tells you how easy or difficult a rock is to scratch. Some rocks are so soft you can scratch them with your fingernail -- chalk and talc are examples. Other rocks, like quartz, are so hard that they can scratch steel. Geologists test hardness by seeing what can scratch what: if your fingernail scratches it, it is soft; if a penny scratches it but your fingernail cannot, it is medium; if even a steel nail cannot scratch it, it is very hard.
Texture describes how a rock feels and looks up close. A rock might be smooth like glass, rough like sandpaper, grainy like sugar, or bumpy with visible crystals. Texture often tells you about how the rock formed. Rocks with large visible crystals (like granite) cooled slowly underground. Rocks with a smooth, glassy surface (like obsidian) cooled quickly from lava. Rocks that feel grainy (like sandstone) are made of tiny pressed-together sand particles.
Color is the most obvious property -- you can see it immediately. Rocks come in every color: white, gray, black, red, brown, green, pink, and combinations of many colors. But here is an important lesson: color alone does not identify a rock. Many different kinds of rocks can be the same color. Two gray rocks might be completely different types with different hardness and texture. That is why scientists always check multiple properties together. A rock that is gray, hard, and has visible crystals of different colors is probably granite. A rock that is gray, soft, and fizzes when you put vinegar on it is probably limestone. Using several properties together is how you figure out what a rock really is.