Rocks are classified according to how they formed. In most cases, we don't see a rock during its formation, so we rely on rocks’ observable clues to infer their formations. Two clues that indicate a rock’s formation are its composition and texture.

• Composition refers to what a rock is made of. The color of a rock can provide a clue to the composition. Fragments of other rocks, fossils, and identifiable mineral grains are also aspects of composition.
• Texture is a description of the rock material. It includes characteristics such as crystal size and shape, number of different grain sizes, and alignment of grains.

  !   Examine these images of rocks. Consider how you might describe the composition and texture of each one. Scroll down the page to begin identifying your own rock samples.

This investigation presents an online key for identifying real rocks (not rock images). On the Web pages, you'll answer a series of questions about a rock to come up with its type (igneous, sedimentary, or metamorphic) and its name. Tools such as a hand lens or magnifying glass, dilute hydrochloric acid, and a nail for testing hardness will be necessary for some of the tests.

  !   Before you start, you'll need a rock sample in your hands. To begin using the rock identification key:

1. Record the rock's sample number on your table.

2. Describe the texture and color or composition of your sample.