Create a Collection That Rocks! (continued)

There are thousands of different kinds of rocks on our planet, each one revealing a bit about its history on earth. Geologists classify rocks into three main groups:

• Igneous rocks. These rocks come from melted rock that has cooled and turned into a solid. Some examples include pumice, granite, and basalt.
• Sedimentary rocks. These rocks are created when layers of sediment, usually carried by water, cement together in layers. Some examples are shale and sandstone.
• Metamorphic rocks. These rocks are created when sedimentary or igneous rocks are buried deep within the earth’s crust and are changed by intense pressure. Some examples are gneiss, slate, marble, and quartzite.

Here was some ways that you and your child can identify rocks.

1. Make observations about the rock. You can identify a rock by figuring out where it comes from or what color it is.
2. Hardness Test: How hard is your rock? Geologists classify rocks by how hard they are to scratch, using a scale from 1 to 10. The softest mineral talc has a hardness of 1 while a diamond, the hardest mineral, has a hardness of 10. You can figure out how hard your rock is by scratching another object against it. For instance, a penny is about the hardness 4. If you scratch the rock on your penny, the rock must be less than 4 hardness, but if the rock can scratch the penny, the rock must be greater than 4 hardness. Experiment by scratching your rock with different objects to see how hard it is.
3. The Streak Test: Rub your rock against a piece of unglazed porcelain such as a kitchen or bathroom tile. Does it leave a streak? You can sometimes identify a rock by the color of its streak. This is more accurate than looking at a rock's color which may be affected by impurities.
4. Cleavage Test: You can sometimes identify a rock by how it breaks, or cleaves. For instance, halite or salt splits into cubes while mica splits into thin sheets.

Identifying rocks can be tricky, but pictures are very helpful. Check out an illustrated rock field guide which you can get at any bookstore, or visit an online gallery such as this one at About.com.

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