Hi,
I am looking to put together my own specific gravity setup and am wondering what everyone uses? Pictures of your setup and any information you can provide is grearly appreciate.
-Daniel
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I’m not going to post a photo bcs my setup is generic and simple. Here’s how to use any scale to get a pretty good SG estimate. My scale is a ~$15 scale, capacity 50 gms, measures to 0.02gm accuracy. You may be able to find one that measures with the same accuracy that has a 100gm capacity.
The procedure is this: use a small, very light plastic cup with enough water in it to immerse the specimen with a little room to spare. A salad dressing cup or a 4 oz clear plastic cup. You need to suspend your specimen on a thread. You can tie the specimen with a tight loop in the thread if it’s irregular enough. Better yet, you can glue the end of the thread to the stone with CA glue. The CA glue bond will last long enough to measure the SG.
- Put your cup of water on the scale and push the tare button to get to zero.
- Immerse the stone in the water completely, but hold it up off the bottom of the cup. Write down total, which is the weight of the water displaced by the stone.
- Lower the stone until it is resting on the bottom of the cup, while holding the thread, now not taut anymore, loosely to keep most of it out of the water. Write down the second reading, which is the weight of the stone and is larger than the first reading.
- Divide the second reading by the first. You have your SG.
If you want to know how accurate this method is, weigh something with a known SG. You could use a piece of transparent quartz, which is usually 2.65, no variance. You could use a synthetic stone with a known SG. Measure a couple of times to get an idea of the error variance of your measurement.
This method is usable in the field and usable with larger specimens with a scale with a larger capacity. I use it with facet rough and with faceted stones whose identity I am unsure of. No need for a classical lab setup.
The late Wm.Hanneman has written extensively about using an SG as the first measurement in gem ID instead of an RI. His book, Affordable Gemology is a great read and a good introduction to many practical procedures of gem ID that aren’t in classical gemology texts.
HTH,
royjohn
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Thank you so much! This is extremely helpful. I will definitely look into that book you mentioned.
-Daniel
use a thin thread for a small stone… thread also displaces water but to a neglible extent as long as it’s thin…you can use a small plastic mesh bag but you will need to weigh it also in air and water to make a correction for the weight of water that it displaces, as it has enough voume to affect the maximum seinsitivity of the scale… your scale has to be sensitive enough to measure grams to two decimal places- .01 grams… calibrating your scale several times with a known standard as suggested by royjohn is necessary to know the variance… that must be taken into account when you weight a stone… the principle is very simple but getting to work, takes some work…
The one caveat is multimimeralic rocks… some stones like lapis have more than one mineral in it, defining it as a rock… often calcite… depending on how much of a lighter mineral is admixed, you will get a different reading…crystals that contain heavy elements that substitute for light ones also have a range in SG, although narrow. That is the reason why gems have a range of SG… garnets in particular…
This test is easy to do once set up and costs nothing to run. best wishes and best of luck…
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Thanks for your practical knowledge.
as a historical aside, the principle of SG has been attributed to Archimedes…which was is eureka moment… Archimedes was tasked by the king of Syracuse to determine the gold content of a crown made for himself. The king suspected that the crown had it’s gold content diluted by silver, by the smith who made it and who had stolen some of the gold provided, but did not want it to be tested destructively… Archimedes pondered the problem… the story goes that he noticed that water overflowed the bathtub that he immersed himself in… the solution suddenly hit him… he yelled Eureka! repeatedly in joy as he ran down the street back to his house… he was so overcome by his discovery that he even forgot to dress and ran down the street naked… Archimede’s principle is specific gravity… the crown was carefully weighed in and out of water…it was lighter than pure gold… the king was enraged at the goldsmith and executed him…
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