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What type of refractometer?


#1

hi,im looking for a refractometer to complete test 4,and ive noticed there are different kinds.Which one do we need?Ie theres analogue ones,digital ones,handheld ones,are they all the same?


#2

No, they are not all the same. The “classical” refractometer has a window for a light source or an internal light source and a prism window on which you place your stone. You read a value from about 1.4 to 1.81 off a scale you view through an eyepiece. The digital refractometer you refer to will give RI values on faceted stones with well polished surfaces, but is actually measuring reflectivity. It’s advantage is that it will read RIs over 1.81, but it won’t give you a value for dispersion or help you determine optic character and sign, which a traditional refractometer will. It should give you a value for birefringence, however. AFAIK there are no hand held refractometers for gems. Those are used in agriculture (brix measurements). You are looking for a “classical” refractometer, which you use with a somewhat toxic refractometer fluid which should be used with some caution, though any risk is minimal with careful handling.

You can buy a cheap Chinese refractometer on ebay for about $100. Be aware that you will need to buy the RI liquid if it is not included with the refractometer. While these work about as well as more expensive models, order one that has a return privilege and test it out on a piece of quartz, which has a rather invariant RI. If your reading doesn’t agree with published values for quartz, the refractometer needs to be returned for one that is correctly calibrated. In return for this minor inconvenience, you’ll save several hundred dollars, at least, over the US, Japanese and German made models. Get the 1.8 fluid, as it is somewhat less toxic than the 1.81 fluid and there are few uses for that extra 0.1 units of RI.

While you want the classical model for learning gemology, many people swear by the digital models for everyday use at about $100 more (on ebay). You don’t need a fluid and they will measure any RI, not limited to below 1.81. You will easily find instructions for using the refractometer on the internet. If you really want to use it to greatest effect, after you’ve learned how to get an RI, learn to test for birefringence and to estimate dispersion and then go on to finding optic character and sign, which is slightly more complicated, but very useful if you want to get everything possible out of the refractometer. If you can do that, using the refractometer along with a good loupe will identify most gems.