Dear Guys,

I am just a beginner and I have problems separating 'corundum and Alexandrite’ and ‘Aquamarine and iolite’, both seem to have overlapping RI, thermal conductivities, color, hardness, SG and biref, etc. Do you have other suggestion how to separate them? Thank you all for your help in advance.

Regards,
Zen

Hello again Zen,

A first and easy thought is to check the pleochromism. Where the stones have their usual colours, the dichroic/trichroic colours of each are very different. However, if the stones are colourless or very nearly so, the pleochromism may be hard or even impossible to spot. But... if a Beryl is truly colourless that it can't be Aquamarine, can it ?

How well do you get on with your refractometer and what reference source (or sources) do you use to look up the 'standard' RIs of stone species? All these four species are anisotropic. This means that they polarize light and have two RIs, one for the vertically polarised light and one for the horizontal. It follows that when the reference table to (say) Alexandrite says 1.746 - 1.755 this is not an RI range but are the two (most usual) RIs exhibited by the species. If the reference source is a good one, it will say more, something like 1.746 - 1.755 (+/- 0.005). The figure in brackets RI is the amount by these RI's have been found, in practice, to vary.

Now, if you look at the entry for Corundum (Ruby or Sapphire) you should find that there is, in fact, no overlap. These have RIs 1.762 - 1.770 (+0.008/-0.003). No overlap and easy to tell apart.

Now look at the figures for Iolite (Cordierite) and Beryl (Aquamarine) and you should find a similar clear distinction.

There is more..... Aquamarine is Uniaxial whilst Iolite is Biaxial. Careful use of your refractometer and a little calculation will tell you this (optic sign also). Read up the method in Read's 'Gemmology' ('Gemology Project' here on GO too, but I find Read's write-up the easier to understand). There is also a similar discrimination possible between Corundum and Chrysoberyl (Alexandrite).

Getting the hang of all this can be a bit of a pain but, once you have the hang of it, your refractometer is going to be your best friend

Dear Kerensky,

Thanks a million! Really appreciate it so much for your response

Dear Kerensky,

I am using a reflectivitly meter which is not able to show that kind og resolution you are talking about

Regards,
Zen

so you MUST purchase a refractometer, learn how to use properly and perform those test again...

ciao
alberto

Advice time. Put your reflectivity meter in a drawer, lock the drawer and forget where you hide the key and rediscover it again in a year or so. The best of them are secondary tools to be used by those who understand. pretty exactly, what they are doing and what the limitations of a reflectometer are. One of these boxes simply cannot replace the need for a well set-up gemmological refractometer. Read this recent thread from start to finish to understand better the reasons for this advice. viewtopic.php?f=11&t=14336.

Sadly, many are mis-sold to beginners as a quick and easy replacement for a refractometer. I know; I was briefly tempted myself when I first started.

If you are serious about learning gemmology, you really do need to own one or at least have frequent access to one. In my view, a good refractometer is essential to the study/ID of cut and polished stones.

What other tools do you presently have?

Dear Kerensky,

Thank you for your clarification, it is very helpful to me
To be honest, I do not owe a wide range of equipments I only have a polariscope, a reflectivity meter, a thermal conductivity tester, a spectroscope and a loupe. By the way, I heard that JEMETER, a reflectivity meter can resolve double refraction of gemstones, it is true? But It is no longer on sale.....

S'OK. We all have to make a start - and an x10 triplet loupe is the very first thing to get. If you have not already got one, you also need at least one good light to use with it (and with your spectroscope). If you are only going to have one light, make sure it has an incandescent bulb rather than a white LED. A mini-Maglite is quite cheap, the favourite pocket lamp of many gemmos - and you may already have one about the house.

On a budget of less that 50 bucks, you can get a mini-Maglite, a battery-operated long wave UV light, a pair of jeweller's tongs and a conoscope (for use with your polariscope). With just these instruments, you can experiment and learn a lot. You also need some books. If money is tight I'd recommend that you buy these second-hand (easy over Amazon). If you do not already have a copy, the book to start with is 'Gemmology' by Peter Read.

You should get a dichroscope too. Get a model built round a Calcite rhomb and not the cheaper 'London' variety. You'll never regret spending the few bucks extra....

With just those tools and a first read through of 'Gemmology' you are ready to take on the gemmo world I'd further suggest that, until you can buy a gemmological refractometer, you concentrate of the study of gemmy mineral specimens rather than cut and polished stones. Mineral samples are (usually) cheaper to buy (and at the very beginning) you want to look at 'more' not 'better' Also the good news is that, until you are ready to splash out maybe 4-500 bucks on a good refractometer, a study of gem crystals in their natural state is not hindered by not having a refractometer!

Another useful instrument is a hydrostatic weighing kit with a balance capable of 0.01ct (nominal) accuracy when weighing stones as small at 0.50 ct. PrettyRocks.com will sell you a neatly boxed set that is (just about) up to the job for under 80 bucks. For a better rig, you can spend a *lot* more money - or make your own - if you are good with your hands and need to save the money. You can read how in 'Gemmology'

If you have a PC/notebook, a copy of the Gemmology Tools Professional software is also heartily recommended. This is a data base of thousands of gem material photos and spectra, plus the tabulated optical and other properties of 500+ gem varieties. Much more too - and all for under 50 bucks! It's a 'best buy' - but only after that copy of 'Gemmology'. See here http://www.gemologytools.com/html/purchase_gt_pro.html

Unless you intend to specialise in the study of Diamond and its simulants, I'd be tempted to sell the reflectivity meter and conductance tester and put the money received towards that good gemmo refractometer which, sooner rather then later, you really are going to need.



Read that thread yet? There's really no more to add. For now, just take that on trust. Eventually, if you want to spend some time exploring the basic physics and mathematics, the certainties of what is and what is not truly possible will be revealed. There are few things as satisfying as finding the answer to a puzzle - and understanding it - for oneself

The best of luck, anyway! As you have questions bring them to the forum. There're always folk to provide helpful answers and insights.

P.S. Re. learning from mineral specimens. This morning I had a lovely, clean and gemmy, 20-odd ct lump of Peridot drop through my letter box and I've just opened the package. Classic termination. Some parallel growth. Couple of inclusions. No detectable magnetic attraction..... A lovely apple green too - so Magnesium rather then Iron then... Wonder what the SG is? Come to that, what's the range of density change in Peridot as Fe is isomorphously replaced by Mg? Don't think that's in the reference tables. Must work it out from the various atomic weights and file the note if significant. Extend the line of enquiry to other stones that can have just a one atom replacement or addition?

Hours of looking, testing, learning and general pleasure to come..... even without that refractometer, microscope etc

Dear Kerensky,

Thank you for your sincere advise and I really appreciate it very much! I am going to follow your advise to purchase the refractometer and database soon. Till then I will come back with more questions

Have a wonderful day ahead!

Regards,
Zen

Dear Zen,
You are surely welcome. And that's good news about your plans re. a refractometer and a copy of GT Pro.

The refractometer I use (following a recommendation in this forum) is this one
http://www.gemproducts.com/Refractometer.aspx.

This one is cheaper and from a little-known (Chinese) maker but it is warranted for accuracy, is well recommended by experienced gemmologists who use it and is offered retail by an EU-based dealer who is a forum member.
http://www.1001raregemstones.com/gem_te ... uments.asp
I'd recommend going for the FGR 002 model rather than the FGR 003. Talk to Max and he will give you a nice discount if you also buy some of his cheaper stones as a 'starter' study pack.

Moving up in price, names to google are Kruess, GIA Duplex and Eickhorst. There is probably no better unit than the Eickhorst - but one really needs to buy in to the Eickhorst system, at least their monochromatic light source.

You will see E-Bay and elsewhere stuffed with ads for unbranded Chinese-made units at between 60 - 90 bucks.Though there are some in this forum that have bought one of these in general, they are not good. If you must buy one of these, pay the few bucks extra to get it from a good reseller who will change it without hassle if you return it because of faulty calibration.

IMHO, the decision priorities should be, in order:
1. Accuracy, accuracy and yet more accuracy.
2. Ease of use, including testing of cabochcon-cut stones.
3. Durability of the hemicylinder.
4. General build quality.
5. Price

Oh, and make sure that the unit either comes with a monochromatic (Sodium-yellow) light source built in - or else you have such a light source you can use with the refractometer you choose to buy.

Good luck! Do let us know here how you get on.
Best wishes,
Kerensky

I have one of these and it is an excellent instrument. I recommend it as well.

Dear Barbra and Kerensky,

Thanks alot for your information and I will get my refractometer soon:) Have a nice day!

Regards,
Zen