Joined: Fri Feb 24, 2006 1:20 am Posts: 2756 Location: Southern California, U.S.A.
What test did you use, Crystal Star? Thermal conductivity or reflectivity? I have a Presidium reflectivity tester and can't get it to I.D. the same white diamond accurately twice in a row!
I was going to suggest a reflectivity test but if the surface of the subject stones has been degraded by irradiation as Richard Wise mentioned, it might not be very decisive. I think Snizzy mentioned that thermal conductivity can be inaccurate due to metallic inclusions but it might work fine on some stones. It's worth a try.
Since some of the black diamonds are on the inside, and others the outside of the curved hoop earrings, they cannot be read on the reflectivity meter; especially the ones on the inside curve of the hoop. So I just grabbed the thermal probe and confirmed to myself that I had tested them when I initially got the machine, and they all tested properly, as diamonds.
I tested everything I could find when I first got the thing, and one pair of those earrings had been an ebay purchase from some years ago that I was concerned might be hematite. or God knows what. So I was relieved that they at least tested out thermally, as what they were sold as.
I agree the reflectivity meter is inconsistent, at best. I wonder if it catches facets at odd angles, giving fluctuating readings on the same stone. Whatever it is, sometimes doing another type stone, and then returning to the first one, will give two consecutive consistent readings. I've had the thermal probe do the same thing, and changing stones and then going back seems to reset the reading, as it were, but with that test the stone likely just needed to cool down before being tested again. But for either one, going to another stone, and later returning to the first, seems to be the key - go figure.
Joined: Tue Dec 06, 2005 11:45 am Posts: 1107 Location: Cape Elizabeth, Maine
hehheh wrote:
Is the inaccuracy of diamond testers on black diamonds a commonly experienced and known fact?
It is commonly experienced - but not sure if it's a known fact.
I had contacted several suppliers of black diamonds and was told that due to the trace elements contained in them - they frequently do not test thermally as diamonds. I have had the same experience as Crystal Star - I own black diamonds and sometimes they test as diamonds and sometimes they don't.
The iris diaphragm is broken on our GIA scope at work, so tomorrow I will take in my own equipment, like I do every Monday when I do all of my appraisals. Hopefully I can get some transmitted light through the stones. I can also put them on my Jemeter Digital 90 to try to make the refractive distinction. If those tests fail - it looks like S.G. will have to be the deciding factor and the stone will have to be taken out of the mounting.
With all of the recent studies on black diamond and the wide range of possibilities of what the inclusions may actually be, for example anything from graphite, magnetite, hematite, garnet, peridot and so on, it makes me wonder how reliable an SG reading would be on it's own.
If in fact you had a black diamond that was heavily included with hematite, which is considerably heavier than diamond it would nudge the SG up significantly, no?
It just seems to me that the readings could potentially be all over the board depending on how heavily included it is and what the inclusions are. Maybe it's enough to get you within range along with your other tests and observations to make an educated guess.
Joined: Tue Dec 06, 2005 11:45 am Posts: 1107 Location: Cape Elizabeth, Maine
Well, fortunately this time I was able to make a postive ID. I was able to get some transmitted light through the pavillion and my Jemeter Digital 90 confirmed diamond with a 2.41 R.I., so no S.G. needed.
Life is good
Last edited by Snizzy on Mon Mar 06, 2006 8:58 pm, edited 1 time in total.
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