I am going to be starting the practical exam of the Gemologist Certification program, but I’m suddenly having difficulties with my refractometer. I got this particular model on Amazon and it has a nice light built into it so I don’t have to worry about using an external light source. I was able to get readings on this refractometer before, but today I’m not getting anything at all. I am trying to test one of the tiny 3-4mm round gemstones IGS just sent me for the exam, but all I can see is a silhouette of the gemstone itself.
Just yesterday I was able to get readings on three of my own cut aquamarines; they are much bigger gems. Today, however, I’m not getting anything on the aquas at all and with the IGS test gem, all I can see is a silhouette of the gem. Could I be doing something wrong? Is the built-in light maybe not so good?
To add to Troy’s list; make sure you are centering the stone on the window, it also could be that the stone is above the index. It may be something like a garnet or zircon which read above 1.81RI.
Thank you for your response Troy. To answer your questions:
I keep the refractometer in its box (it’s padded) when not in use, so I don’t think anything has been damaged.
I do use RI fluid, but perhaps not enough. All the advice I’ve seen on this site as well as several others says to use just a little drop of it. Should I try using a little more perhaps?
The scale is clearly visible to me when I turn the light on.
I just can’t see the shadow lines anymore for some reason. I can see the outlines of the stones on the scale, but that’s it. It’s definitely weird.
Does the size of the stone matter? The ones IGS sent me are smaller than the window so much of the light passes around them. To be candid, I was kind of surprised they were so tiny.
Thank you Daniel. The one stone from the IGS set that I tried might be a demantoid garnet, but I’d be surprised if they’d send one of those in the test set. Then again, it IS really tiny! What worries me is the fact that I wasn’t getting readings on my own larger aquamarines either yesterday when I HAVE gotten RI’s in the past for the same stones. I.e. last week I got consistent RI’s for them, however when I tried them again yesterday I couldn’t see the shadow line at all.
I just did an experiment with my refractometer, which has its own yellow light source probably similar to yours. I have a very small tanzanite 3.4mm x 2.4mm oval. It took me a while to find the shadow. Initially, I was seeing the stone image in the eyepiece, so I had to move my head closer to the instrument until the stone image faded away.
The shadow was elusive at first and it did take a larger dot of RI fluid. I started out with a really tiny dot and had to increase to about 1/3 size of the stone’s table. I then had to move the stone up and down on the window slowly. The shadow was very weak at first, but then I rotated the stone slightly to get better definition on the green line and shadow.
My test stone has a very shallow cut (almost too shallow) and not very well portioned.
This is actually quite helpful! I will use your procedure and see if I’m more successful. It does sound like we have similar refractometers and I suspect I may need to do a lot more adjustments as you described.
My own vision issues may be at least partially at fault for this difficulty too. I have an impairment in my dominant eye from an illness I had long ago; it’s a blind spot that makes some of these tests trickier to perform. (a little hand held spectrometer is almost impossible for me to use.) Thank you so much!
I have vision degradation to blue light, due to nature of my work environment, which requires a lot of time in front of a computer screen. A common issue for many that creeps up on you over time unless you use the “anti-blue” feature on the monitors. (Which didn’t exist when monitors where nothing but glorified TV CRT tubes…)
I have to use what is called “averted vision” where you have to look off-center from the intended focus point, in order to see strong blue light. So what I do… is initially focus my eye on to the side of the scale, to observe the “shadow/green line” outside of my center focus. It takes a few times for my brain to zero in on it… but it works. Once I have identified the terminus, I can then use regular vision focus.
I intend to get a better quality Refractometer that has a larger ocular view eyepiece… it will cost a bit more, but it definitely will help.
One thing that occurs to me regarding seeing the shadow line in refractometers is that it might be easier to see the whole rainbow that you get when you use daylight or a full spectrum light source rather than the yellow light in the instrument that is meant to be a close substitute to monochromatic sodium light. So if your instrument allows you to use a white light, try that. You can then zero in on the yellow part of the spectrum and read the RI on the scale that is closest to that. It’s a more approximate reading than using the yellow light alone, but the white light provides a larger target to see and can be made brighter if you have a very bright light source. I don’t have one of the refractometers you’re discussing, so I can’t say how bright your yellow light is, but I’ve had yellow light sources that are dimmer than using a bright flashlight and the polaroid one uses to isolate one of two readings in birefringence also cuts down on the light intensity. So using the white light to see whether you get any reading at all first might be a good idea. Once you know where to look, you can try the other light source…hope this helps.
Another comment I would make is that you need only a very small drop of fluid if you can be sure the gemstone facet or polished inspection facet on a rough piece is right on top of the droplet. I’ve used a head magnifier to see what I’m doing so that I get the measuring face right on the droplet. If the specimen is transparent, often you can see the droplet spread out between the gem surface and the prism and know you’ve got it right. Might even see the liquid squish out at the side of an opaque stone using the spot method.
Be sure to clean your prism with alcohol and a clean tissue before beginning until it is “squeaky clean.” And dry off the alcohol, right? Prolly you know this, but it helps to go thru all the steps carefully when you’re having trouble, which we all do at times. -royjohn
When i was going to buy a refractometer a gemologist i know told me to get one with a detachable light source. I dont remember their exact reasoning for this… but i assume it has to do with being able to use other light sources, including natural.