many questions about Taaffeite and the equipment used to test it

Hello, i have recently got a small waterworn Taaffeite crystal, it came with a gem identification certificate which shows the various instruments used to test it, it was tested with Polariscope, Refractometer and Microscope, it was not tested with Dichroscope or UV test.
There is quite a lot i would like to find out about these instuments, i have no understanding at all of these technical sounding things so if anyone can explain what these things do, in very basic terms i would really appreciate it!
I would like to know also if the tests it has had done are enough to confirm it as Taaffeite as i have heared stories of people cutting spinels in a certain way so as to fool some testing equipment, and also some spinel can have double refraction due to internal strain.
The back of the specimen looks like its been sawn flat and polished, i have seen this on other taaffeite specimens to, and have read thats its to help with the identification, why/how does the polished window help? can they do the tests to crytsals which have naturaly flat and smooth faces?
I hope soemone can help, i posted this on another website and some fo the replies i got i thought were quite rude, like questioning the authnticity of the certificate and apprently how taaffeite and muscravite are tested is quite boring, maybe to them it is becuase they already understand how its done but i am a complete novice, i can indetify some minerals by hardness tests and SG tests and a lot of the commonor minerals i can id just by sight but i would really like to know more about the technical equipment, not that i'll ever own such things but its just nice to know!

Polarscope:
[www.gemstonebuzz.com]
Refractometer
[www.gemstonebuzz.com]

Microscope - for viewing something at magnification - not a particularly definitive id instrument.

The above instruments will help to determine the optical properties of the stone, but optical properties can vary for an individual mineral species and optical properties can overlap between different mineral species. These properties can narrow down the range of possible minerals, but don't necessarily limit it to a single species.

Doh, for some reason it didnt click when i saw microscope written on the certificate, sorry, i do actually know what one of those is!

I'm sure others with more knowledge than I will follow up, but to my understanding, the critical testing to distinguish taaff from spinel is the exact measurement of the refractive index and the observation of double refraction. The polished window is commonly made on a stone suspected of being a taaff so that those optical readings can be properly made. (A flat surface is needed for the instruments to transmit and read the path of light into and out of the material. If the crystal already has a smooth flat surface, then it may not be necessary to polish one.)
I've never heard of spinel displaying anomalous double refraction due to internal strain, but it could be possible and rare. Some diamonds do it, and so do some garnets. But anomalous double refraction usually looks and reads different than real double refraction (so I hear), so it should still be possible to use this property to distinguish.
As for using a regular single-refracting spinel to "fool" the instruments, I don't think it is possible. It is possible to cut a uniaxial stone (like taaff, corundum, etc.) so that the optical axis is perpendicular to the table, thus making a reading of RI taken from the table look like a single RI from a singly-refractive stone. But readings taken from other facets would give away the trick. (On a uniaxial stone with two RIs, only one is read if it is taken down the axis, and two from any other direction.) But there should be no way to test a stone that has ONE RI and make it look there are TWO.
Congrats on your taaff. Post a photo? One of the stones I have yet to add to the gem crystal collection.....

Jason,
Could you describe your water worn Taaffeite crystal? Is it transparent? Probably not very or they would have cut a stone out of it already. The fellow who wrote up your report used three standard gemalogical identification tools and they might be good enough to get the job done. Maybe. The microscope would not be much of a help and he is relying mostly on test run by his contact refractometer. You lift the lid of the refractometer and there is a small glass flat lying window there which you put a tiny drop of a liquid there and then you carefully put the flat surface of your stone face down on the glass window, move it around a little bit with to make sure there is a good contact, close the lid, turn on the light (a somewhat monochromatic source is best and usually comes standard with the refractometer) and you look through the view finder and read out the refractive index(s). If the stone is uniaxial or biaxial you may need to take more than one reading to try and get the maximum high and low RI. This will give you the birefringence which you can then use look up on your chart to get an indication of what stone you have. If you stone is fairly transparent you might then be able to use the polariscope to resolve an a uniaxial or biaxial optic figure and to determine the optic sign if you have a quartz wedge handy. Often gemologists have on a set of blinders that limit their identifications to the few minerals that are on their A & B stone charts and are not able to consider the full range of minerals that are really out there. But in most cases, you could probably make an identification with these instruments. Taaffeite, according to the literature sometimes exhibits a false biaxial nature, but you could ask the guy who gave you the certification to explain how the readings he got prove that the stone is really Taaffeite. His explanation of how he arrived at his identification might provide a further assurance as to how accurate his identification might be. Usually the use of a contact refractometer requires a flat polished surface to get good refractive index readings. Only rarely do natural crystals have a crystal face flat enough for this purpose.

Rock Currier
Crystals not pistols.

Here is my Taaffeite, its not an amazing specimen!
[www.mindat.org]
In the description it says its light purple which i couldnt see but since i wrote that i have noticed it is pale purple in certain light, i have taken further photos which capture the colour betetr but the pic i posted on mindat is more well focused so i will leave that as it is.
I have been after Taaffeite for ages now, the same thing happened with painite, i thought i would never be able to get such a rare mineral, then a few months later suddenly it doesnt seem as rare anymore!

Thank you all for your help, certainly more constructive and helpfull than the replies i got on another website.

Jason
Im not to sure how easy it is to check double refraction on such an included stone. If you like you could do you own test to check this. If you two pairs of polaroid sunglasses,(2 pieces of polarised fim will also do) make up a cardboard holder, to hold the sunglasses about 2cm apart, then hold them up towards the sun , rotate the stone between the glasses, if you see a the light thu the stone going from light to dark and back again, its most likely doubly refractive, taafftite.
There was en excellent article on Sri Lanka taaffite, in Gems and Gemmology, about 15 years ago, you might find interesting , if you can find it,
In the Ratnapurra market in Sri Lanka, I once say a star taaffite, for sale .......Greg

This one does have a chatoyant effect, not a star, but definaley chatoyant

Not sure how a two-year old thread found itself listed in 'New Discussions' where I found it - but it's still an interesting one with many useful pointers.

Jason, you clearly take an intelligent interest interest in how to determine the species/variety of a specimen using only non-destructive and moderately priced equipment. Perhaps I should start by saying that it can't be done in every case but the occasions when one needs to reach for quantitative chemical analysis (and the services of an expert and experienced analytical chemist) are fewer than one might think.

If you have not already found it, get your self a copy of 'Gemmology' 2nd edn by Peter Read. This is both cheap and simply the best introduction in the English language to to the testing and identification of gem quality minerals (about 500 varieties that are, with a few exceptions transparent or translucent). This introduces all the instruments about which you wish to know more and provides expert explanation of their functioning and use in clear and simple language. But this is just a very first step. If the bug bites you, you will quickly find your way to creating a small reference library, most of the 'foundations' for which you will now find short-listed at [www.gemdat.org] .

David, I can't agree that a good microscope in skilled hands is not an excellent tool in the identification of many gemstones. The key issue is one of identification of inclusions which can not only be unique to a variety (e.g. Emerald or Moonstone). This requires a sufficient database of reference information, a good indication of the locality of origin of a specimen. And there is more.... What is true, however, is that not all stones are communicative under the microscope but that said, for me no examination is complete without a careful examination under the microscope (given that my interests are confined to specimens of not greater than miniature size and to non-destructive testing).

The correct identification of Taaffeite is a classic example of the folly of jumping to the conclusion of an ID based on the results of just one or two types of simple testing of physical and optical properties. Nature's workshop is chaotic and turns out some strange combinations. This means that practical testing (any one test) is less than certain in giving an ID. Also, for any one variety, there is frequently more than one variety that will return an acceptable result to the same single test. So identification becomes a process of elimination, with the application of a series of tests and sources of information to slowly reduce the possibilities of a correct ID, eventually to no more than one variety.

For Taafeite, the key differentiation of it from Spinel is the isotropic nature of Spinel. It's a given in good practice with a gemmological refractometer that if a specimen only shows one RI when tested on a given facet, a second test with the refractometer in contact with a different facet should be carried out. In the case of a rough specimen, testing with a refractometer may not be possible. In which case examination in a polariscope must be conducted with particular care, ensuring that during the examination, the stone is rotated about all the three dimensional axes.

Jason, It may be my imagination or an artifact of the photograph, but check the shadow of the stone. The transmitted light seems to be slightly lavender or purple.

There seems to be a recent rash of specimens that look like Jason's - and at dirt cheap prices. These may be no more of gem quality that testing out as Corundum makes a Ruby. I can't say more because I have never yet examined one of these whitish 'iron-stained' stones.

Here are two run-of-the-mill gem quality Taaffeites:

Fantastic gem info, and vast amounts of info on gem ID instruments is available here:

[www.gemologyonline.com]