gemex vs. hca

[SOCKS]

I bought a diamond that scored 0.7 on HCA. The diamond scored High using Gemex, Brillianscope. Is Gemex correct? I am looking for a diamond that can score "very high" on Brillianscope. How trustworthy is Gemex reading?

 

[Rhino]

HCA is a prediction based on averages of 17 facets and assumes perfect optical symmetry. BrillianceScope is a live analysis of the diamond and the light interaction between all 57 facets. While I am always interested to know HCA scores, they do not take precedence over BrillianceScope results. I''ve tested quite a few stones that get great HCA scores but take a hit in live analysis. The only other light measuring device I know of that makes an assessment without seeing the stone is the DiamCalc Software but if you import a model into it based on an actual scan of a diamond it also happens to agree with B''scope results as well.

Does the diamond you''re looking at have crown angles <35 degrees and pavilion angles =<40.6 degrees? Curious.

 

[niceice]

Date: 2/6/2005 11

9:31 PM
Author: Rhino
HCA is a prediction based on averages of 17 facets and assumes perfect optical symmetry. BrillianceScope is a live analysis of the diamond and the light interaction between all 57 facets. While I am always interested to know HCA scores, they do not take precedence over BrillianceScope results. I''ve tested quite a few stones that get great HCA scores but take a hit in live analysis. The only other light measuring device I know of that makes an assessment without seeing the stone is the DiamCalc Software but if you import a model into it based on an actual scan of a diamond it also happens to agree with B''scope results as well.

Does the diamond you''re looking at have crown angles <35 degrees and pavilion angles =<40.6 degrees? Curious.

Ditto. With all due respect to Garry Holloway and the HCA which is a great tool for pre-screening and eliminating the possibilities, the results are based upon the average measurements as opposed to taking all of the contributing measurements into account and thus it can only project the potential of the diamond and not yield actual results. The problem lies in the fact that the average measurements are the result of a range of measurements which can yield different degrees of visual performance / light return depending on how tight or broad that range of measurements may be. For instance, the average pavilion angle might be 40.8 degrees which is likely to produce really nice light return if the spread of the high and low measurements is "tight" like within a few tenths of a degree, say 40.6 - 41.0 but would probably not be so great if the spread were greater like 40.3 to 41.3 degrees - both would yield an average pavilion angle of 40.8 degrees, but the difference in light return is likely to be great. The same concept holds true for the crown angle measurements... So use the HCA to narrow down the possibilities and then look to the Gem Ex results to narrow the results further... By the way, there is nothing wrong with a rating of High / High / High or High / Very High / High, etc. the reality is that once you cross over into the region of High you would probably be "really" hard pressed to tell High from Very High without some real coaching and a set of bionic eye balls... The bar scale used by Gem Ex presently is a bit skewed as far as how it may be interpreted by people, the largest range in terms of percentage lies between the categories of low to medium which account for about 85% of the scale and then the range from High to Very High accounts for the remaining 15% of the scale, so once you cross into the range of High and above, you''re working in the top 15% of the range of possibilities so you''re off to a really great start in terms of what you''re considering...

 

[perry]

I would like to toss a cautionary note into this discussion.

Gemex is looking at light return from some very specific angles, which may match up with certain light return patterns that a 57 facet round brillinat diamond automatically generates.

However, there are other great looking diamonds out there where the diamond light return patterns do not line up with the gemex reading spots, and gemex will give them a lower score. For example I am told that 8* is one of these.

My view of the situation is, that gemex only gives great scores to only a portion of the grouip of great looking diamonds. So if your diamond has great gemex scores it will be a great diamond. But there are many more great looking diamonds out there.

Gemex has to the best of my knowedge refused to let an independent lab test its machine to determine how representative and fair its readings are (ratios of false positives, ratios of false negatives, etc).

Both Rhino and NiceIce are right about the meaning of the HCA. It only gives potential, which is why Garry H. tells you to look only for stones with HCA below 2 and then to look at the diamonds, if possible, to see how they look. A diamond with a HCA of 1.9 might look much better than a diamond with an HCA of 0.5.

Best of luck with your search for a great looking diamond.

Perry