Identify rocks by their properties
Q: I have a clear rock with blue streaks. Do you know what kind it is-Ashia
A: Like you, I go on hikes, run across an interesting rock, and wonder what it is. To answer the question, we must turn into
geologists and look at the rock properties (for example, hardness, streak, specific gravity, fracture, and luster).
Right: [Brooke Weston City Technology College] Rock 1
Already, you can see my difficulties. I don't know your rock's properties, except its color (clear) and streaks (blue). But that
doesn't give enough information to answer your question. Instead, I'll give you the tools to analyze your own rock.
First, work through a project from Brooke Weston's rock ID. See the table below. That exercise will give you an understanding of the procedure. As an
example: pick Rock 1 from the project's rock list. Here are the tests you go through and the results that allow you to identify Rock 1:
| Test |
Results |
| 1. Close examination-look at it, maybe under a magnifying glass. |
It's got close grains of medium size and is off-white color. |
| 2. Acid test-dump some hydrochloric acid on it. Be careful with the acid. |
It fizzles and gives off carbon dioxide gas. |
| 3. Scratch/hardness test-scratch it with: your fingernail, a copper penny, a
piece of glass, a penknife blade, and a steel file |
A knife scratches off rounded grains.
|
| 4. Consult the rock key |
The chart splits depending on whether or not the rock fizzled under
hydrochloric acid. It did fizzle. The chart then gives two fizzle choices:
limestone or marble. Read the description of both and look at their pictures.
Then pick the one closer to your Rock 1 specimen. It looks to me that
limestone is a clear winner. |
That's the idea. Geo Man's mineral and rock charts are more extensive. Run tests on your real rock the same as in the online project and consult Geo Man's
charts if necessary to identify your rock. Good luck!
By the way, I did ask your question of Ask-a-geologist, Lawrence Anna of the US Geological Survey. Here's his reply: "Most clear rocks, or probably in this
case, minerals, are either quartz, calcite, or gypsum. The blue streaks are probably an impurity such as a derivative of cobalt or maybe copper."
(Answered by April Holladay, science correspondent, July 18, 2001)
Further Surfing:
Rogue Community College: Geo Man's mineral and rock charts http://jersey.uoregon.edu/~mstrick/MinRockID/MinRockIndex.html
Ask-a-geologist, USGS http://walrus.wr.usgs.gov/ask-a-geologist/
Books:
The Practical Geologist, 1992, by Dixon and Bernor ($13)
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