Hi, what do you recomend for a Specific Gravity tool? Also, what equipment is necessary(or helpful) for identifying faceted gemstones?
Hello,
You maybe able to find a specific gravity instrument for gems by going online as I believe I have seen them before whilst looking for other things. For measuring refractive Index you will need a refractometer and standard RI liquid. Proper usage and interpreting the reading or readings requires instruction and skill. Any one of us, including myself, will be happy to help you navigate reading refractive indeces correctly.
Best,
Brian Hudson GG (GIA)
Hi. Thanks for the information. I should have specified, but I already own a Refractometer and am very familiar with using it. I also have a dicroscope and loupe. Among other small tools.
SG is something you measure with a scale and some water. There is no “specific gravity tool” that you can view thru or press up against a gem to get SG. Get a scale accurate that displays to 0.01ct (accurate to about 0.02ct) and has a “tare” function, a small plastic salad cup and some water and a piece of string to tie to your gemstone rough. Then look for my post on this forum about measuring specific gravity. For loose gemstones, you’d need to figure out a cradle that is nearly weightless. If the gem is mounted, you are SOL and have to use a refractometer for RI and birefringence and dispersion. Or use a polariscope for SR vs DR and, if you are very, very good at it and the stone isn’t bezel set where you can’t turn it round and round, you could get optic sign and optic character…but nobody does this, even tho’ it’s theoretically possible. So $80 to $100 for refractometer, which you already have. Learn some more gemology and you will know what instruments you need for a rough and ready kit. Read A. Matlins book for what instruments to get and how to use them. QED. -royjohn
2 Likes
Thank you
SpG is Achimedes’ Principle at work… it’s the weight of the stone in air versus the weight of the stone in water. A sensitive balance that weighs a stone in air and then in water is all that you need. Roy John made specific recommendations on how to set it up. As a historic note, Archimedes was tasked by the King of Syracuse to nondestructively test the gold content of his crown. The king suspected that the gold smith had stolen some of the gold and made the crown out of an alloy containing less gold than he was given. Archimedes pondered the question… his solution came when he took a bath and the water overflowed when he got into the tub. That was the EUREKA moment…the myth says that he ran home naked in the street shouting Eureka… I got it!.. The WEIGHT of the VOLUME of water that his body displaced is the basis of SpG…the weight of the stone in water will be equal to it’s volume displacement of water. Hence it will be lighter in water than in air. Geologists use this technique as a field test or easy bench test to start identifying a mineral crystal. Same will go with a refractometer with clear mineral crystals. Refraction of plane polarized light along a given crystal axis will vary by the axis, giving an even more useful tool for mineral crystal identification. With a rotating stage, the angle of extinction gives the geologist a way to identify very small mineral grains within an igenous or metamorphic rock in thin section…by counting about three hundred points and identifying the mineral grains, a rock can be classified by MODAL analysis… NORMATIVE analysis requires grinding the rock uo and chemically analyzing it, which is more intensive.but also give a lot more information. This is off topic somewhat but is explanatory for the scientific principles in rock and mineral identification. Gemology doesn’t exist in a vacuum. I think of it as applied mineralogy.
Another way to test for SpG is by using differing concentrations of Clerici solution. Clerici solution is a 50/50 mixture of thallium malonate and thallium formate. It has a SpG of 5 grams/cc at 90 degrees C and an SpG of 4.25 grams/cc at room temperature. By using serial dilutions, a gemstone or mineral will either sink or float depending on it’s density which is related to SpG. In addition, Clerici solution’s refractive index varies linearly with its concentration. Stones that have a given refractive index will “disappear” when immersed in Clerici solution with the same refractive index… cryolite has the same refractive index as water and “disappears” when immersed in water. Clerici solution testing is no longer in use, although it still can be bought on line… Thallium is extremely toxic !!!
Thank you, this is very interesting.
no problem… nobody knows about clerici solution now days… too toxic for general use… requires gloves, and no skin contact… although available, safer testing methods are available…SpG measurements and refractometers have made it obsolete…even though it can test both SpG and refractive index at the same time…
I wanted to add my experience in creating a cradle for holding gemstones for consistently determining specific gravity. I am often working with small faceted stones or in rare cases, even faceted semiprecious beads and it took many trials and a great deal of time to find an acceptable “cradle” to hold the stone.
My solution was to use a length of 28 ga silver wire, formed into a loose spiral to create a funnel shape. The tightest part of the spiral works as a “closed” end at the bottom. At the top of your funnel, leave an extra length to give you something to hold on to. It does take a little trial and error to get the least amount of wire that will hold your gemstones. I ended up making 2 different sizes - one for 1 ct and less, one for larger stones. If done correctly, this method displaces “no” water during measurements - at least not even to a hundredth of a gram. I realize this all might sound unnecessarily detailed, but I found it made a huge difference in getting accurate readings. Once you find a system that works for you, its really quite easy… just a few calculations. Good luck!
@LauraR97800 Thank you for the information. Could you by chance send a photo of your specific gravity setup?
@DanielB66512. So sorry… its currently packed away. But, the only tools you need are an excellent digital scale that will weigh things to a hundredth of a gram that has a tare function. Instead of a salad cup, I use small glass beakers (just a preference). And the cradle I described. One last item: a calculator.
I was going to post a specific link to a video, but it seems that is against the rules. In any case, search you tube for videos on " Specific gravity for minerals". There are several to choose from. Watch a few and it will be very clear to you shortly. Good luck.
No problem, thank you so much.