- Virginia Johnson
Rocks come in all shapes and sizes, but what kind are they? You can’t ask them, but sometimes, if you know how to listen, they’ll tell you anyway.
The shape and size of a rock doesn’t tell you much about what it’s made of.
Big rocks break into smaller rocks all the time. But there are other things to look for that can give you their I.D.
First, the color is often helpful. Quartz is yellowish-brown on the outside but often shiny clear-white inside. Granite is often gray speckled with white and black, while calcite crystals are often a transparent pale yellow, almost as clear as glass. Precious stones often vary only in color – if a crystal of the rock called corundum is red, it’s a ruby, but if it’s blue or any other color, it’s a sapphire!
Hardness is another important clue. Some rocks are much, much harder than others. If a rock can scratch glass, it’s harder than glass. And if it can scratch another rock, it’s harder than that rock. Diamond is the hardest rock – it can scratch anything, but nothing can scratch diamond! Softer rocks include talc, pumice, and the gypsum inside the wallboard in your walls.
Is the rock heavy for its size? Then it has what’s called high density. A dense rock weighs a lot even if it’s small. A less dense rock, like pumice, is as light as Styrofoam. You can pick up a big block of pumice easily, but don’t try lifting a piece of basalt the size of your head! It weighs more than you do.
If color, density and hardness don’t positively identify your mystery rock, you may be tempted to smash it with a hammer. That can actually help reveal another important rock property – fracture. The way a rock breaks is key to identifying it. Slate, for example, breaks into flat sheets when smashed, which is why it’s convenient to use it for blackboards. Calcite’s cleavage breaks it into prisms, and fluorite breaks into eight-pointed shapes.
Armed with your knowledge, you can identify hundreds of different kinds of rocks. And unlike birds, you don’t have to sneak up on them! Though you can if you want to. They won’t mind.
Books for Rock Hounds
Diamonds and Gemstones by Ron Edwards and Lisa Dickie.
Learn about the "cursed" Hope Diamond, the origins of these amazing jewels, what they're used for, and how they're cut to show off their true beauty.
Eyewitness: Rocks and Minerals by R.F. Symes
All about the rock cycle: creation, erosion, and sedimentation, plus lots of photos and information on their uses.
National Audubon Society First Field Guide. Rocks and Minerals by Edward Ricciuti and Margaret W. Carruthers
Detailed full-color spreads help beginning naturalists observe and understand over 150 types of rocks and minerals.
Rock and Fossil Hunter by Ben Morgan
More than 30 fun activities for rock and fossil collectors!
Rock and Mineral by John Farndon
A guide to the uses and history of rocks with lots of great photos.
Rocks on the Web
Interactive Rock Cycle Animation
A very cool look at the way rocks form.
The Mineral Gallery
Examine minerals by class (elements, oxides, carbonates, etc.). View the chemical formula. Lists properties and histories. Pictorial.
The Rock Identification Science Project
A virtual rock lab where you figure out the types of rocks. This site teaches proper identification techniques and includes a good chart to help.
Rock Identification Tables
Once you're comfortable with the rocks you find all around you, check out this table for identifying more unusual rocks. You will need to know whether your rock is igneous (formed by fire), sedimentary (formed by sand or clay, may have fossils), or metamorphic (an igneous or sedimentary rock that has been changed by fire or pressure, or both).