What is specific gravity of rocks?
It is the density of a substance relative to water.
For example, magnetite has a specific gravity of 5.2.
This means the weight of magnetite is 5.2 times that of an equal volume of water.
The most common equation for the specific gravity (SG) of a mineral is this:
It's pretty simple to work out:
1. Weigh the mineral in air (literally put in on the scales like normal)
2. Suspend the mineral in water for the second weighing.
Legend has it that is was Archimedes who discovered that the weight of the mineral in air minus its weight in water is equivalent to the weight of the water displaced by the mineral.
So why bother measuring specific gravity of minerals?
Specific gravity is actually one of the most important, constant, and quantifiable properties of a gem or mineral.
So in your effort to identify an unknown stone, the specific gravity, if known, can narrow down your search to just a handful of possibilities.
Here is a useful table with a list of specific gravity measurements of minerals.
Must my mineral specimen be pure?
Yes and no.
No, because you can theoretically measure the specific gravity of any solid using this method, whether it be a chunk of plastic, a rock containing a varying amount of minerals, or a pure mineral itself in the form of a crystal.
But yes, your mineral specimen must be pure if you wish to compare your measurement to standards of specific gravity for those minerals, standards of which were developed using pure mineral specimens.
In summary, the purer your specimen, the more accurate will be your specific gravity measurement.
What the best way to measure specific gravity of rocks?
When you can use the old dustbin filled with water and hang your scales from the top, is anything else worth looking at?
It depends how useful the specific gravity of core measurement is added into your dataset.
Most of the time because it's not a really simple process and the scales take 90 seconds to settle, it's put in the 'too-hard basket'.
"Send it off to the lab, they'll sort it for us."
How useful would it be for you to have this info in real time?
Check out this explainer video on how you can perform your own SG readings on site, in your own time.
Manual Specific Gravity / Bulk Density Method
Automatic Specific Gravity / Bulk Density Method
Have any more questions about specific gravity or the physical weighing stations?
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