The HCA is easy to use. You just simply plug in the information in a diamond lab report and it outputs a score.
This is what you get from an HCA result:
If you were ever wondering how to get 4 excellents on the HCA, then you should have simply clicked ‘go’ with the default values given as you load the page.
If we go back to the homepage and just increase the depth % by 0.4 to 60.4.
As you see that is all that’s in between 4ex and 3ex and 1vg.
Well most did some more testing and with all other things equal to the default, a 61% and 62.5% depth gets a HCA score of 0.5 and 0.7 respectively; not much difference at all.
For a beginner, the HCA can be a pretty useful tool. However, it has been criticised quite a lot by people in the trade so we will go over some of the criticisms about the system.
Over-reliance by Consumers
First, even with all its disclaimers highly visible, many consumers still place significant weight on a HCA score. Most are one of the consumers that almost made a purchasing decision based on the HCA score.
This is of course not the fault of the HCA system or its inventor because it clearly says on the first page that the proper use of the HCA is to:
“use it only to reject likely bad performing diamonds to narrow down your final selection.”
On the second page it says again:
“Having found a diamond that scores well, you should employ an expert appraiser to examine the stone. If you decide not to, then at least compare the diamond to others and/or view it through an ideal-scope.”
As the idealscope was also developed by Gary Holloway, the above statement could be seen as a marketing gimmick in order to sell more idealscopes! Regardless, the idealscope is undoubtedly a useful bit of kit that especially helps out the untrained eyes.
The reason many consumers still place such weight on the HCA is not surprising because it makes a very big claim. If you find a diamond that receives a score of less than 2.0 then you have “eliminated known poor performers (more than 95% of all diamonds).”
This first part is seemingly simple, but we will see if this claim holds up in a bit. The part we found confusing was this:
“Your own personal preference may be for a diamond with an HCA score of 1.5 more rather than one with a lower score of say 0.5.”
If you’re worried about the numbers, our understanding of the second statement is that a perfect HCA score is 1.0 and a variation of 0.5 either way cannot be qualified into whether the diamond is better because that is a matter of preference.
Garbage in, garbage out
The problem is that the HCA calculation relies on only 5 number inputs. These 5 numbers don’t take into account all of the facets a diamond.
The numbers on the lab report themselves are problematic because they are first averaged, then rounded; not to mention that there are margins of error in the measurement of those numbers. If for any reason the numbers on the certificate don’t represent the diamond well, then the HCA won’t be able to pick up these problems.
The HCA also makes it clear that it doesn’t take into account of polish and symmetry.
The point so far is to read the label before you use. Use the HCA as a rejection tool and not a decision making tool.
Here’s the thing. When we realised the limitations of the HCA tool, it suddenly became a lot less useful for me in my search for my perfect diamond. Alot of people never use the HCA any more as there are more effective methods of rejection in diamonds. The general advice is to stick with the recommended proportions in the "how to pick a diamond" tutorial, then if you find a diamond that passes those test and yet you’re still not confident enough or want to make sure, then you can run the numbers in the HCA.
The below test is using a 62.5% depth because the depth and culet don’t seem to affect the score that much.
For a 34/41 Crown Angle/Pavilion Angle, the HCA score for the table facet between 53 and 57% was: 1.6 ex ex vg vg.
For a 35/40.6 Crown Angle/Pavilion Angle, the HCA score for the table facet between 53 and 57% was: 1.0 ex ex ex vg.
It’s clear that within the recommended specifications for crown angle, pavilion angle, and table percentage, changing the table does not affect the HCA score.
If you are interested in what what a (Table/Depth/Crown Angle/Pavilion Angle) 55/60/34.5/40.8 gets, the HCA score is: 1.2 ex ex ex ex.
If you limit the stones you look at to excellent cut stones, and if you follow the recommendation specifications in terms of proportions, then the HCA is reduced to basically comparing crown angle/pavilion angle relationships.
So since you are reading this and are likely to be stricter in terms of your cut requirements. There really is no need to bother with the HCA since you are already equipped with the knowledge to reject poor performing stones.
This is not to say that the HCA doesn’t have its uses. If you are looking at older cuts that are graded ‘good’ or ‘very good’, and you don’t have a strong understanding of how all the facets work together, then it may be useful to stick these into the HCA. Don’t expect to try to find excellent scores though, remember that it is used to point out red flags.
For example, the HCA does a good job telling you about potential ‘fish-eyes’ (reflection of the girdle seen through the table) and about very thin girdles.
The HCA is also good for telling you whether your diamond fits into the range of a BIC, TIC, or FIC.
BIC stands for Brilliance Ideal Cut, TIC stands for Tolkowsky Ideal Cut, and FIC stands for Fire Ideal Cut.
Gary Holloway states on his website that the requirements for a BIC is a crown angle less than 32.5 degrees and a FIC is a crown angle greater than 35.5 degrees.
To summarise, the HCA can be a very useful tool for identifying problems with a diamond. Just remember to use it for what it was designed for and that you should not base your buying decision solely on the HCA score.
Does the HCA hold up to its claims?
Well, it sort of does. The HCA does what it says on the box, but it’s not a magic calculator. Remember that it only assesses light performance and doesn’t tell you anything about optical symmetry. In many cases, you can achieve the same results quicker by following the range of specifications that I set out in the "picking a diamond" tutorial.