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Specific Gravity Instructions - Step by Step
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| Description |
Instructions |
| Figuring out Specific Gravity |
The tools that you use are entirely up to you. Regardless of your setup, the principles will remain the same.
(1) Weigh the stone in air.
(2) Weigh the stone suspended in water.
(3) Put these two weight values into a formula that gives the specific gravity.
Now as with any simple set of principles it is possible to completely screw it up. That is why I wrote these instructions! I will attempt the
impossible feat of anticipating every way that someone can misread, misapply and outright mess up on these two principles! :)
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| Weigh Stone in Air |
There is no levitation or magic carpets involved. Instead it is called "weigh in air" to contrast with the next step which is "weigh in water."
More later on the rock swimming lessons.
(1) Get a scale with at least 0.01 carat accuracy.
(2) Turn scale on and wait for the reading to go to zero.
(3) Put your clean and dry stone on it.
(4) Record the weight of the stone.
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| Weigh Stone in Water |
Make sure your stone is not water soluble! Please don't call me if your halite beauty has transformed into salt water.
(1) Get a scale with at least 0.01 carat accuracy.
(2) Turn scale on and wait for the reading to go to zero.
(3) Setup whatever tools you are using so that you can suspend the stone in water and place it on the scale.
(4) Press the "Tare" button on your scale so the reading will go to zero.
(5) Gently place your stone onto the surface in the water such that the stone is completely submerged and suspended.
(6) Usually you should give the setup and the water some time to stop moving or shaking.
(7) Record the weight of the stone.
| Calculate Specific Gravity |
Take the air weight of the stone and the water weight of the stone and put them into the formula shown below.
( AIR WEIGHT )
(AIR WEIGHT) - (WATER WEIGHT)
For Example: If the air weight is 1.50 carats and the water weight is 1.12 carats then the formula will be as below.
( 1.50 )
( 1.50 ) - ( 1.12 )
The Specific Gravity equals 3.95.
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| What is Specific Gravity? |
Specific gravity is the density of the stone relative to the density of water. So the specific gravity of water is one (1). In the example above, the specific gravity is 3.95.
This means that the stone has a density that is 3.95 times greater than the density of water.
As you may have guessed, the density of water is not absolute. If you didn't guess this, then oops, sorry to confuse you! The impurities in water can affect the density, because, well, they ain't water! Also, the temperature of water changes its density. Ice is more dense than boiling water which is still more dense than steam. So if you live in say, Sahara or Siberia, adjust your calculations please. The rest of you can relax so long as you use nice clean room temperature water even if room temperature varies from 40 degrees to 90 degrees fahrenheit. I really hope you don't have to work in conditions worse than this! From 40 degrees to 90 degrees fahrenheit the density of water will change by 0.005 grams / centimeter3. This will affect your calculations by about zilch. If you want to worry, look at your scale. No, not YOUR scale, the gem scale!
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| What Did I Do Wrong? |
The Specific Gravity came out to 198.32 and you know you don't have a Black HoleTM gemstone from Columbia Gem HouseTM. Yes, they copyrighted that one. Just in case.
Here are some possible causes for the crazy values you're getting.
(1) Your scale has fossilized, consider a proper burial.
(2) Your water has lead, arsenic, and other chinese spices making the density otherworldly. Actually those impurities which make the water heavy, making your mineral seem too light, not heavier in the final calculation.
(3) Your water is over oxygenated (read bubbles) and this makes your mineral seem heavier than it really is.
(4) The specific gravity contraption you are using might be improperly suspending the stone.
(a) The suspension device is leaning on the edge of the water container, which will lower the water reading and make the stone appear less dense.
(b) The suspension device is so heavy that it increases the chance that the suspension tool will affect the reading.
(5) When you tared the scale, the suspension tool wasn't settled and the tare didn't take well.
(6) Your table is unsteady or the suspension tool is unsteady causing your reading to be inaccurate.
(7) Stop drinking!
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| Tips for Improving your Readings |
(1) Get the air weight first, then the water weight. That way you will not have to add drying to your process. Yes, this has come up!
(2) Use Distilled Water
(3) Make sure your stone is clean. Particularly when getting a reading for minerals, if it is in matrix or has some residual matrix on it,that will distort the reading.
(4) Use a really good scale. Getting consistent accurate readings is very important. When I use a cheap scale, I take several readings and use the one in the middle of the pack.
(5) Make sure your table and the suspension tool are extremely stable. Stability is critical to getting an accurate reading.
(6) The suspension platform must not be touching anything other than the scale platform in order for the weight to be accurate. Okay, it will be immersed in water, but it can't be touching the water container.
(7) Use a calculator. Don't hurt your brain trying to do math. It ain't natural!
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| Further Reference |
Check out Wikipedia for more water facts!
Water Defined by Wikipedia
Look at this Wikipedia page on Specific Gravity to see why I created my page. Keep it simple!
Specific Gravity Overcomplicated
The finest gemology reference site on the web,except for ours, of course.
GemologyProject.com
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| CYA! |
I am no expert! I use these tools as best I can, but I am not a graduate gemologist by any means. I try to keep it simple as much for me as for you, so please use this as a starting point and when you get in deep, consult your books and experts. Thanks! |
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