Hello everyone, another long-time-lurker with a question.
I've not had my refractometer long and have been testing stones with known RIs to make sure I'm doing it correctly. All has been going ok, but this morning I tested a couple of morganites (I suspected one coule be something else) and got unexpected results.
I read in The Gemology Project that the RI for morganite is 1.572-1.6, but both my stones just sit around 1.585-1.599 which is quite different. I feel really stupid at the minute and have no idea what I'm doing wrong. The readings were so close together at some points that I had to double check they were not singly refractive!!
Post subject: Re: Refractometer giving odd results...
Posted: Sat Feb 16, 2013 11:19 am
New to the Forum or The Quiet Type
Joined: Fri Feb 08, 2013 4:32 am Posts: 3
Hi Barbara. Yes, I managed to get a reading from a pavilion facet from one of the stones, (the other would not give a reading), and managed to get two shadow edges this time. One was 5.8-5.9, and the other was 1.67-1.7. Still confused!
Don't stare yourself blind on the numbers too much. Gemstones form under differing conditions and nature therefor produces results that fall into a range of possibilities rather than exact numbers. Take note of the fact that just an RI doesn't lead to a positive ID most of the time. A well executed RI measurement gives you 3 things:
1. A Refractive Index (RI) which will fall into a range. Let's take 1.544-1.553 as an example.
2. The difference between those two values gives you your Double Refraction (DR). In this case 0.009.
3. Since you have performed 4 measurements, turning the stone each time, you have a table of results from which you can determine the Optical Character and Sign (OC&S). In this case probably something that looks like this:
One shadow edge moves, so the OC of the material is uniaxial. It is the lower values that move so its T is negative... 1-
Armed with those three weapons we then venture into our tables. Here's the one I created for my students, it lists the most common stones (and some more):
Let's start by nominating a bunch of candidates... Our RI was 1.544-1.553 so we select our possibilities:
As you can see I've taken a rather broad selection, just in case my measurement isn't entirely accurate or I'm holding a freak of nature. I now know I'm dealing with either chalcedony, Iolite, Sunstone, Quartz or Scapolite.
Next I look at my DR: 0,009. This DR allows me to rule out chalcedony. It wouldn't give me a DR since it's polycrystalline. Sunstone is becoming less likely a candidate because its DR is on the low side but I'm not going to dismiss it since I may have been messing up my third decimal estimations.
So... 4 candidates left:
Then finally I have an OC&S: 1-. AHA!
Scapolite!
Now... in your specific case, an RI of 1.585-1.599, you don't even need those 3 steps... Beryl is the only candidate when you look at the list of RI ranges...
You state:
Quote:
I read in The Gemology Project that the RI for morganite is 1.572-1.6, but both my stones just sit around 1.585-1.599 which is quite different.
which tells me that you haven't got it clear yet that those values are RANGES within which your test results may fall.
On DR: the values given are the MAXIMUM DR you might get and depending on the mineral, its orientation and your RI measurement you may or may not have gotten that maximum RI.
Finally, on the OC&S: with uniaxial materials the optical sign is usually easily determined but with some biaxial materials the sign may be more difficult to acquire. There are even materials that can be both positive or negative (some feldspars and chrysoberyl for instance)
Post subject: Re: Refractometer giving odd results...
Posted: Mon Feb 18, 2013 5:15 am
New to the Forum or The Quiet Type
Joined: Fri Feb 08, 2013 4:32 am Posts: 3
Wow, thank you so much for your detailed reply Tim!
I'll try not to get so worked up over the numbers now! The way some people talk though does imply that the refractometer is the be all and end all of test equipment, but yes, I hope I know better than that at least.
Then finally I have an OC&S: 1-. AHA!
I'm sorry, but would you mind explaining this to me? Or is this what is covered in the Optical Sign section in the link you gave me? I have done a practical Gemology course with Gem-A, and am now doing the foundation cert, but I haven't seen this mentioned anywhere. Maybe I'm just getting in too deep!
When one shadow edge moves during your measurement while the other stays put you are dealing with a uniaxial mineral (meaning this double refractive mineral has just one direction through which it appears to be single refractive). When both shadow edges move you are dealing with a biaxial mineral (meaning it has two directions through which it appears to be single refractive).
So, character wise there is just those two possibilities. One optic axis - uniaxial. Two of 'm - biaxial.
The sign determines which edge moves. With uniaxial stones we look at which edge moves. If it's the lower values it's negative, when it's the higher values it's positive. With biaxial stones you have to determine which edge moves the most. If it's the lower values it's negative, when it's the higher values it's positive.
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