Rocks and Minerals: Top 10 Questions

May 2008

Thanks to Scott Hughes, Department of Geosciences, Idaho State University; and Virginia Gillerman, Idaho Geological Survey for the answers.

1: How many different kinds of rocks are there?

There are probably at least a couple thousand different kinds of rocks, but every one is unique. Rock is composed of a mixture of minerals, and if you've looked around outdoors you might even notice the granite in central Idaho looks a little different from one place to the next. It is estimated that 300 names help sort rocks into categories between sedimentary-, igneous-, and metamorphic-type rocks. (From Caleb in Mrs. Hooper's class at Gooding Elementary)

2: What makes certain rocks valuable?

Rocks are usually valuable either for their beauty and their decorative value, or else for a valuable element that they contain. Gold is the one that you might be thinking of, but many times we get valuable things from the earth that we may not appreciate, such as certain rocks that are used in jet engines and for lubricants and are important ingredients in many specialty steels. And so it's the use of something, whether it is sand or gravel, or gold, that makes it valuable. (From Tessa in Mrs. McCamish-Cameron's class at Cynthia Mann Elementary School)

3: Why are rocks so hard?

Rocks are hard because of the way their atoms are bound together. For example, you might have a rock that's made up of a mineral that has carbon. Carbon is one of those elements that, if it's bound a certain way, it makes a very soft mineral called graphite. But if it's bound a different way it can be so hard it becomes the hardest mineral known to humans, which is called diamond. Rocks are also hard as a result of the minerals they contain and how those minerals are bound together. For example, granite can be very hard because its minerals are intergrown and locking very tightly. Whereas a rock like sandstone that might not have its minerals packed together too tightly could be very soft. (From Rachel in Mrs. Whitesell's class at Gooding Elementary)

4: Can diamonds melt?

Yes, they could, if you could get it hot enough. While we are not sure offhand what the melting temperature of diamond is, any substance has a solid, a liquid, and a gaseous form. And that's essentially what melting is, going from the solid to the liquid. On the other hand, if a diamond were melted, it would be worthless after that because it would just be a blob of carbon liquid, and there's no way to make it back into a diamond, that's for sure. (From Taylor in Mrs. Gris's class at Horizon Elementary School)

5: If diamonds are the strongest rock, what is the second strongest?

Diamond is a mineral. And so the strong - the hardest - mineral is diamond. The next hardest one would be corundum. Well-known varieties of that stone would include the gem ruby, which is of course red, and emeralds, which are green. There is basic non-gem quality corundum, which is maybe not very common but not an uncommon constituent of typical metamorphic rocks like we have in northern Idaho. And it has a hardness of nine on what's called the Moh's (rhymes with toes) Scale of Hardness, which is the most common method used to rank gemstones and minerals according to hardness from 1-10. (From Kyle in Mrs. Gris's class at Horizon Elementary School)

6: Why are some lava rocks red and others black?

When lavas flow, like the lava flows one might see in Hawaii, they might be red because they're hot and glowing, but as the lava cools, the lava will become a black or an orange, or a red or brown, depending on the oxidation state. Iron, for instance, if you take your dad's hammer and you throw it out in the yard and leave it out in the rain and so forth, it gets rusty and it takes on a different color than the nice shiny steel when he brought it home from the hardware store. When lava cools, some of the gasses react with the iron in the lava to change the state of oxidation of that iron and so much of it comes out red. (From Jesse, a homeschooler in Filer)

7: What is the biggest gem in the world?

There are some gems that are quite large. The Hope Diamond is the most famous. It is a couple hundred karats cut, but of course it was probably several, maybe a couple of thousand, karats before it was cut and facetted into a cut gemstone. On the other hand, there are quartz crystals in museums that are almost gem quality, and some of those you could get one that was a foot long, and six or eight inches across. Some of the largest gemstone crystals come from Brazil, and you may have seen some of those in rock stores or in a museum. (From Montana in Mrs. Hammond's class at Camelot Elementary School in Lewiston)

8: How do rocks form in the ocean?

In the ocean rocks can form in two general ways. They can be what we call chemical precipitates, like a limestone, that might result from crystallization from dissolved carbonate parcels - particles that collect together, and then drop to the bottom of the ocean and form as layers of limestone. The other way, which is probably easier to see, is, for example, on a beach where you have a sandy beach, and over time if the ocean sort of retreats, that sand layer will get buried by other layers of sediment, and eventually over millions of years compressed into what could be a very hard rock. In the Boise foothills we have sandstone that was probably a lakefront beach at one time, and we can have finer grain sedimentary units as well. (From Sebastian in Mrs. Colburn's class at Endeavor Elementary School in Nampa)

9: How do rocks get their shape?

Rocks are formed by three different types of processes, and depending on the process, a rock will have properties making it softer in one direction, and harder in another direction. Or it might have some texture which we call foliated or lineated or layered, etc. As rocks break, they break along planes of weakness, so during the weathering process, for example "mechanical" weathering, the rocks are broken up and the shape depends on how strong they are in various directions. On the other hand, a rock may be washed over with water and eroded down so the surface of that rock might be changed just due to the erosion. It all depends on the property of the rock itself and how strong it is in one direction versus another. (From Joshua, a homeschooler in Boise)

10: How do you find the age of a rock?

There are two main ways geologists determine the age of a rock. The first way, of course, is with fossils. Fossils represent old animals or plants that evolved in certain directions, and paleontologists have studied that over the years and determined what the relative ages of those plants and animals are. More recently scientists have used radiometric age dating of rocks - that's a consequence of the decay of certain unstable isotopes of some elements. Rocks decay at a uniform consistent rate. By measuring certain isotopes within the rock and doing some mathematics you can calculate an age of the rock. So like we think the earth is about 4.5 billion years old, we can determine ages of rocks that are as young as 50,000 years on the Snake River Plain, for example. (From Alexandra, a homeschooler in Boise)

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