Joined: Mon Dec 14, 2015 5:45 pm Posts: 24 Location: UK
Hi everyone,
I'm loving all the resources available on the site and I'm currently practising with my refractometer. Previously I used to just take the one set of readings, the one with the biggest disparity I could find, but I'm now learning to take a full set of readings to identify the optic character of the stone in question.
This leads to the beginners question: I have obtained the results below, from what I believe to be a Tourmaline. Please could someone explain why the first and the last readings appear to be mirror images of each other? I didn't want to just dismiss it as a mistake and ignore it, in case I'm not grasping something fundamental. I've interpreted the figures as giving a Biaxial stone with a Birefringence of 0.021, but am totally lost as to which way to jump on optic sign. And of course, the book is saying Uniaxial -ve.
Post subject: Re: Beginners question on Refractometer
Posted: Fri Apr 20, 2018 11:46 am
Site Admin
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Hi Dan. Let's talk about technique for a second. This series of numbers isn't telling us the whole picture. May I suggest you read through our article on the Gemology Project: https://www.gemologyproject.com/wiki/in ... ractometer
When using the polarized plate, you should get 2 readings in every position. Write them both down. In a doubly refractive, uniaxial gem, like tourmaline, one of the values will remain the same throughout your readings. The other changes. The greatest difference between the 2 numbers will be the birefringence.
This is thoroughly explained in the link I provided above.
Post subject: Re: Beginners question on Refractometer
Posted: Fri Apr 27, 2018 7:16 am
Gold Member
Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
Your very first reading should be labeled like this :
1.624 1.645
not :
1.645 1.624
You should always put the smallest figures in a first line and the biggest on the second line. Therefore, no confusion would be possible. If you write the figures in the correct order, you can see that all the figures of the second line are the same or nearly so (just a small difference between 1.645 and 1.643 but negligible - probably a mistake of your reading). And then it matches with a uniaxial negative stone, which the tourmaline is.
In your case, the facet you study is parallel to the C axis of the stone. When the 2 figures of your reading are identical the light goes right through the C axis.
Usually a set of 4 readings is enough, within a half turn of the stone on the window of your refractometer. After a half turn you cannot get new figures nor a higher birefringence.
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