MadSci Network: Earth Sciences

Re: Is crystal a rock?

Date: Mon Aug 14 00:15:18 2000
Posted By: Sarah Fretz, Undergraduate, Biology, New Mexico Tech
Area of science: Earth Sciences
ID: 957233706.Es

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  to see if anything looks familiar.  Happy 

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