|MadSci Network: Earth Sciences|
Dear Brittany Crystals are most definitely rocks, but not all rocks are crystals. Crystals and rocks form in similar ways, either with heat, or time and water or oil. The biggest difference between rocks and crystals is that crystals are more pure than rocks. You see, both rocks and crystals are made up of minerals. Minerals come in all shapes and colors and act differently when they’re treated with chemicals or broken with a hammer. How these minerals form can change from rock to rock, but here again comes the issue of purity. If a mineral is mixed with lots of different minerals, or some dirt, or even some water when it’s made, chances are that this mineral will be a part of what seems to be a plain old rock. If by chance the mineral is heated slowly and evenly in the depths of the earth (and later pushed to the surface over millions of years), it may become a crystal like the ones you found. This is how quartz crystals form, and a lot of the most common sand is just very small, worn down quartz crystals. Crystals can also form from water. Water in the ground and in the oceans and streams of the world washes over rocks almost constantly. It can dissolve small bits of rock and soil and later redeposit the similar bits of rock in the same place. An example of this is a stalactite or stalagmite that you would see in a cave. These pointy columns formed from water that dissolved a mineral from the local rocks and then dripped from the cave ceiling to the floor over hundreds of years. You might not be able to get to a cave to see this (I sure hope that you get to sometime, caves are pretty cool), so ask your parents if you have “hard” water. Hard water is just regular water that has some calcium and magnesium minerals dissolved in it. When the water evaporates from your sink faucet or the inside of a tea kettle, it leaves the minerals behind in the form of very small and messy crystals. One way to know if what you have is a crystal or not is to break it. Having a crystal form is what is known as a “physical property”. If you were to rip your paper into a billion little shreds, it would still be paper, just really small paper. If you were to burn the paper, it would be something else, since the ability to burn is a “chemical property.” Pay attention to what the new broken bit of rock looks like. (Geologists call this “fracturing” the rock, but they really just like hitting them with hammers too.) Where it broke, is it a clean break, or is it dusty? Did it break off to a smooth face or something that looks like glass? Do you see any new colors? Here comes the key: if you can break this broken rock and the result looks like the first break, you probably have a crystal. If all the bits and pieces look like diamond shapes, or perfect cubes, or it all instantly turns into a huge mess of glittering sand, you definitely have a mineral and you probably have a crystal. If you could break your sample rock a few thousand times, you might be able to look under a high-powered electron microscope and see a miniature version of your crystal. If you looked under the microscope and saw bits of junk in your crystal, then your tiny crystal wouldn’t be perfectly shaped, and those flaws would show up in your bigger crystals. The big crystals would be oddly-shaped and each time you broke your sample, it would make different looking rocks. Then it would just be a rock. A crystal is just a rock with really well-sorted atoms that fit neatly together in a regular pattern. If you were to throw a few hundred of the same size balls into a box and give it a shake, the balls would eventually find a nice pattern to fall into, filling all the holes nicely. Crumple up some newspaper in there and your nice pattern would have a few lumps in it. This is just a simplified description of the nature of the molecules in a crystal lattice. Once you figure out if your rock is a crystal or just a rock, check out a mineral guide to find out what’s in it. My favorite hand guide so far is The National Audubon Society’s Field Guide to Rocks and Minerals. It’s pricey, but once you get to know how it works, it’s well worth it. Since you’re at the computer right now, swing by http://mineral.galleries.com to see if anything looks familiar. Happy Hounding!!
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