A refractometer for gem identification was mentioned in another thread. Can anyone share your experience with this instrument? I am interested in testing gems that are set in jewelry, not loose gems. A quick research shows a wide range of prices, about $100 - $1,200. There are ones that use a liquid and others that don't need the liquid. Any recommendations or problems to watch out for? I have a Presidium II which is helpful, but inconclusive for many stones. Thanks!
The digital ones (that don't need liquid) have the same accuracy issues as the Presidium thermal meters, although they can be useful in the field for ruling out glass. Get one with a built in sodium light source and a CZ table (not glass), a good one will last a lifetime if you are careful with it. Decent ones in the UK are around £400 and up. The liquid they use is toxic, so read up on it before using. Edit: you have to be able to put the gem table flat on the meter surface, so they are no use if the setting is proud of the gem. Also the gem needs to have good polish, chips and scuffs can cause accuracy issues.
I've got an early version of one of these https://www.ebay.com/itm/220566475907?hash=item335ac95483:g:d7kAAOxy4dNSxHbs Mine had a little torch for a light source but these days the light is built in. You will also need refractive index liquid.
So, I have been watching a few you-tube videos, and I had a long chat with a knowledgeable person at Kassoy. I will share what I have learned. The reading of the shadow lines on a traditional refractometer seems like it could be difficult, especially with mounted gems. I asked the guy at Kassoy the difference between the $100, $500, and $1200 refractometers, and he said mainly it was the clarity and ease of reading these shadow lines. I am not very nimble with this sort of thing, and I do question my ability to use this instrument in a way where I would feel confident with the results. The Kassoy guy suggested I consider the Gem-n-Eye Digital Refractometer. You put the stone on the sensor, and the machine gives you an RI reading and 1 or more possible stone IDs. After looking into it, the machine is not really reading the refractive index, but rather reflectivity, which gives readings very close to the refractive index. As Ce BCA noted, the problem is, several stones will have similar reflectivity readings. With a traditional refractometer, as I understand it, the difference between the high and low readings on a doubly refractive stone will enable a positive ID, but the digital refractometer doesn't measure this. I think in combination with my Presidium and what I can see about the stone color that I could sometimes get an accurate ID. For example, the Gem-n-Eye could give a reading of 1.698 and say that the stone is peridot or tanzanite. The presidium reading and the color of the stone would make this pretty easy to distinguish between peridot and tanzanite. Others would not be so clear cut. Anyway, sorry to be so lengthy. My conclusion is that I don't know if I will buy either a traditional or digital refractometer. Still pondering if I could learn how to use a traditional refractometer with any confidence, or if the digital instrument would produce enough information to make many positive IDs. Both seem difficult to use with mounted gems, thought the digital one I think would be better in this area. The Kassoy guy did say that while they don't like to take returns, they would allow a return if I tried the Gen-n-Eye and didn't find it useful.
Most of the problem with refractometers and loose jewelry is putting the lid down but if at home simply do it in a moderately dim/dark room and leave the top up. That way you can hold the jewelry on the refractometer while measuring it. As for needing as sharp line use a monochromatic light source. This kit is relatively cheap. https://www.ebay.com/itm/320768149360?hash=item4aaf458770:g:v0oAAOSwtutcV54L
If you get one you will be disappointed, go and read the professional gemologists forums and see what they say about it. Any slight scratch, flaw or dust will throw off your readings. Repeatability is poor, ie you get different readings each time you put the same stone on. Also look at their videos, they have perfectly polished stones to work with. Presidium II is useful as it works well in the field to give you an idea then you can check at base with more accurate kit.
One of the main differences is the table, expensive ones should have a CZ table which is much tougher and scratch resistance.
Thank you both. More to contemplate and read. I was looking for one with a cz table, and only saw that for ones being sold in Europe, though I did not do an extensive search.
I found this gem society forum thread on the Refractive Index Fluid interesting: "https://forums.gemsociety.org/t/question-about-unknown-ri-fluid/5659/3"
