Do I need high tilt brightness, BOG, and high ray trace numbers for a good design?
The simple answer to these questions is no, not really. Are they good things to look for in a design, maybe… It depends on what you like and want from a design.
Let me explain. The first thing that you need to know about gemstone designs, or picking one out to cut is…
All designs are a compromise, none of them will be prefect and you almost always have to give a little bit one place to get a little bit somewhere else. What does that mean? Well in our case to get a better tilt brightness (brightness as the stone is rocked, in one or more directions), the designer may have to use a different angle than would be optimal for a higher light return. Less face up brightness more tilt brightness.
Remember that the over all design and design goals (that the designer is trying to create) is almost always going to be a compromise of the various elements… refractive index, design, shape, color, size, tilt, face up and so on.
So where does BOG (is a handy program that helps show what a design will do, given certain conditions, like tilt…) and tilt brightness come in?
Well I think that there needs to be a few distinctions made.
First – In low refractive indexes there is not a lot that can be done to get a high degree of tilt brightness. Low refractive indexes, in almost all designs just do not work all that well when tilted.
Yes, there is some improvement that can be made (and there is nothing wrong with trying for improvement), but the improvement is usually small and to get them other things often have to be changed, often to the lowering of the design objectives. Like I said (above) there is always a trade off, I bet you have heard that in life before.
Remember that most natural gemstones that are available rough for a hobby or even a professional cutter on the market are in the lower refractive index range. Basically I would say refractive indexes 1.67 and down are what I would call lower refractive index and generally the most common type of gemstone rough that can be bought.
Quartz (1.54) – Amethyst, Citrine, Clear, Smokey.
Feldspar (1.56) – Sunstone, Bytonite.
Beryl (1.56) – Aquamarine, Emerald, Morganite, Goshenite, red Beryl.
Tourmaline (1.62) – All types.
Topaz (1.62) – All types.
There are a few exceptions to this rule, like Zircons (1.93) and occasionally a person might get lucky and find a nice Sapphire (1.76). But in general there is not a lot of commercial gemstone rough that is available to cut in higher refractive indexes. With the exception of Garnets (1.73), which really are very limited by their color, saturation, and availability in lighter colors of any size (especially in red, orange, red/purple). I have not covered every type of material but you get the idea.
Basically if you are cutting natural gemstones then the designer is limited to what can be done in the refractive index of the rough and the design goals for the design that he/she is trying to achieve.
Second – In natural gemstone rough, the choice of design(s) is often limited by a lot of different factors. What is the best yield? For example when cutting an expensive piece of rough, the yield would be a very high consideration when choosing a design. Second to tilt brightness for sure, and maybe to over all brightness, depending on the particular instance/stone.
Third – There is of course a lot of other reasons that the choice of design can be limited. Shape of the rough? Color orientation of the rough? Dichroism of the rough? All these factors and more will impact the choice of design that can be used and the design’s performance.
Fourth – What does the design look like? Sounds obvious, right? It is to some extent, but remember that to get the most out of a piece of rough and then be able to set it and sell it… There are some basics that need to be accounted for, most of these design factors will need to be considered before high tilt brightness.
All of the above and more impact the design, designer and finished gemstone. A computer program will not take all these things into account, that is up to the designer and cutter. The program cannot differentiate between any of these various circumstance that occur when actually cutting a piece of gemstone rough in real life. This is where the experience and sometimes just luck come into play.
An experience that I had years ago comes to mind. A girl friend of mine was a music nut and played the violin… She found out that there was going to be a contest at the local college for technical music. She promptly dragged me to the “concert”. I went (like I had a choice?) hoping that the music was going to be worth listening too (I doubted it, but could not wiggle out), or at worst that I would earn a few “brownie points” that I could trade in when I needed them later (I need as many brownie points as I can get generally).
Well lets just say that the music (I use the phrase loosely) sounded to me like all of them were killing cats and torturing their instruments. I expected them to jump up (well I hoped anyway) and smash their instruments against the stage like rock stars at any time (alas I was not that lucky). It was pretty much like finger nails on a chalk board… and I was not allowed to stick my fingers in my ears (I was told that was simply not done in polite company, there went the brownie points).
All the musicians were some of the best around, and technically they were absolutely tops (or so I was told). The point is that while an expert could recognize and appreciate (not sure I believe that one…) what the musicians were doing (I hesitate to call it music).
There is probably not a single person that is not a musician that would even call it music… And not only would the average person not want to listen to it, they would probably run from it, and not care how technically good is was. Once they were out of screeching range of the violins, I bet it would take a gun to get them back in hearing range.
What is my point? How does this experience apply to faceting?
My point is… a few degrees (or even a fair amount) of tilt brightness one way or another, might be interesting to a few people, or us faceters.
But only an expert might be able to see the improvement if it is slight and even if it was 15 or 20 degrees (which in low refractive indexes is not likely, but possible in higher indexes) most people that are not stone cutters will not know the difference.
Most people buying a gemstone could care less, same with over all brightness, at least as a number on a ray trace is concerned. To some degree, people picking a design for their piece of rough (see above design limitations and trade off’s) will not care a lot either and I am not sure they should.
I have a few points here. Is the design attractive as well as good performance? Does it suit the rough gemstone material that it is cut in? Is the rough and design it is cut in sellable? Most modern designs meet these goals and will cut quite nice stones.
Very few people can see a few degrees tilt difference in a designs and by the time the stone is set in a piece of jewelry, I seriously doubt that anybody could tell the difference. Remember when you set a stone it will change the appearance quite a bit depending on what it is set in and how.
What is important to a customer buying a gemstone and a faceter cutting one? The appearance of the finished gemstone.
By this I mean, color, brightness to some extent, the shape of the design, the uniqueness of the design, and the reflected play of the facets in the gemstone.
What does the stone look like? Is it pretty? Does it have that WOW or POP that we all look for in a gemstone? Of course price and ease of setting are factors too… at least when selling stones.
In the end the only important question is… (at least to me)
Is the stone pretty?
If tilt brightness can be gained and the design can be created true to the design goals (at least with in reason). Then by all means I think it is a good idea to consider tilt brightness and I do. If the design has to be changed significantly to gain a few degrees of tilt brightness, then I do not think it is worth the changes in most cases. especially in a low refractive index gemstone where at best the changes would only gain a few percent anyway.
But I certainly would not design for tilt brightness or just face up brightness, and be willing to sacrifice the design goals and appearance of the design.
It is all a trade off in the end and I think that most modern designs do pretty well all things considered and the best designs manage to strike a good balance of all of these various factors.
So my advice is consider all the factors, then choose a design that you think will work best for what you want out of the rough.
Choose a design you find pleasing and something you will not only enjoy cutting but something you can and will enjoy after it is cut. Have some fun.