Questions: Properties of Rocks

Question 1

You have two rocks. You scratch one with your fingernail and it leaves a mark. You try the same on the other rock and your fingernail cannot scratch it. What property are you testing?

• A. Color
• B. Weight
• C. Hardness -- how easy or difficult the rock is to scratch
• D. Temperature

Answer: (C)

Scratching tests hardness. A rock that can be scratched by a fingernail is relatively soft. A rock that cannot be scratched by a fingernail is harder. Geologists use a hardness scale (called the Mohs scale) to rank minerals from 1 (very soft, like talc) to 10 (very hard, like diamond). Hardness is one of the most useful properties for identifying rocks and minerals.

Question 2

You can typically identify a rock just by looking at its color.

• T. True
• F. False

Answer: False

Color alone is not enough to identify a rock. Many different rocks can be gray, white, or black. For example, both granite and limestone can appear gray, but they are very different rocks. Scientists use multiple properties together -- hardness, texture, luster, and more -- to identify rocks accurately.

Question 3

Describe two different properties you could observe about a rock without using any special tools.

Model answer: You could observe its color (is it gray, brown, white, speckled?) and its texture (does it feel smooth, rough, or grainy?). You could also describe its shape (round, flat, jagged) or whether it looks shiny or dull (luster). These observations can all be made with just your eyes and hands.

Rock identification starts with careful observation using the senses. Before using any tools, simply looking at and touching a rock reveals its color, texture, shape, and luster -- four useful properties that help narrow down what kind of rock it is.