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Recommended faceting designs for larger emerald rough

I recently acquired this parcel of stones which is 94% Zambian emerald. It includes (by my standards anyway), quite a number of stones I consider to be large emeralds. The one I have questions about in this post is the one at the far upper left of the group. It’s pretty much a long rectangle as you can see, and it weighs 13.62 carats. It has very good color and clarity, and the junk on the outside is very much limited into the surface.
I’m hoping that some of the members with far more experience than myself can answer a couple of questions, and offer some advice. First question is, can a faceting diagram intended for a 1.54 refractive index be used on Emerald without making changes?
Next, I’ve read many times that typical emerald cuts do a good job of maximizing the finished stone weight, but do a terrible job of enhancing the brilliance of the finished stone. Do you agree with this? This piece of rough has a 1.6 to 1 L/W ratio, and very good depth. Can someone please recommend a possible faceting design for this Stone which would maximize its value?

I have included a side view, and an end view if that’s helpful. The photo which includes the penny, is the best representation of these emeralds true color.
Thank you for any help you can provide.

PS: if anyone is interested in some of these emeralds, or any of the many other stones I have, including sapphire, spinel, tanzanite, tourmaline, ruby, and many colors of garnet, feel free to contact me.
Thanks again

Hi LKB,
While I am a total nut on good facet design and light return, for an emerald of good quality and this size, I would go with weight retention over light return. Color and weight are the operative factors here. The only reason to try to boost light return would be if the stone was going to turn out too dark. If not, I would try for max weight. This might involve using a higher crown, if that’s an option, a slightly thicker girdle, or maybe even a slightly deeper pavilion. None of these will necessarily affect light return a lot. While you are going to be close using a 1.54 design for a 1.58 stone, in general I would optimize for 1.58…but in this case, I would plug in the angles I was thinking of using and see how the stone looked in GemCad or GCS. In appraisal guidelines, a light return percent of 70 to 100% is tops, so a traditional emerald cut may work out OK with no deductions for light return. While there are a few designs for 1:1.6 L/W ratio that are close, I think the trad emerald cut retains more weight. If I were you I would enlist the services of a Master Cutter who could optimize the cut as needed and give you a professional judgement on what to do. I use a guy who designs a cut specifically for my stone if it is worth enough. His name is Jason Delk and you can find him on Facebook. There are, of course, other people who do the same, but I am not personally familiar with their work. -royjohn

Thank you so much! That’s exactly the type feedback I was hoping for. Emotionally I would like to squeeze as much light return out of it as possible, But from a business standpoint, which of course is more important, I want to maximize the value, and your suggestions make perfect sense. Was that a specific faceting design you referred to? If so, where would I find it?

Thank you again

LKB

When faceting you either cut for brilliance or you cut for colour, you don’t want brilliance to visually drown out the rich colours of something like an Emerald. Then you cut for carat retention and create or follow a design which will maximise yield.

Cutting for colour is basically a principal of keeping facet numbers low and this should provide a good balance of interest and colour. High facet counts produce too much brilliance. Beryl has a critical angle of 39 degrees, Pavilion main of 43 & Girdle of 45, Crown main of 42, Girdle 47 and Star 27, so your design should work around these.