Best shapes for highly dispersive gems

[AustenNut]

For gems that are really dispersive like sphene, sphalerite, zircon, etc, what are the best shapes that showcase the fire? I'm assuming that round is a standout, but are there any others?

 

[T L]

For gems that are really dispersive like sphene, sphalerite, zircon, etc, what are the best shapes that showcase the fire? I'm assuming that round is a standout, but are there any others?

Not necessarily a shape, but a type of faceting. I love Portuguese faceting on highly dispersive gems, and a high crown with a small table also helps. I'm sure a lapadary can give better advice than me, but that's just my experience. Some gems are so highly dispersive, it's very difficult to turn that off, but I find step cuts are not typically good for getting lots of fire.

 

[Michael_E]

Highly dispersive stones will show good fire in any shape...as long as their body color is not too dark. If you want to show maximum dispersion then you need a high crown, small table and relatively fewer facets. Smaller facets can be O.K., but broader facets give wider individual flashes and so the colors look more intense and have a bit broader color width. This is best seen in old mine cut diamonds which have tall crowns, small tables and very broad facets.

 

[AustenNut]

Thanks for the feedback!

 

[chrono]

I find that any shape seems to be fine as long as it has a high crown, small table and chunky faceting, much like the antique cut diamonds (OMC, OEC and some transitional cut diamonds). Also, stones with lighter tones seem to do better. For example, I have several blue spinels and noticed that those with lighter tone flash like the 4th of July fireworks while the deeper blues are just intense blue (but not as much of a light show).

 

[yssie]

from a purely numeric perspective
1. the longer the path through the stone, including internal reflections, the more material/opportunity for a refracted & dispersed ray to spread
2. the greater the angular difference between two facets/virtual facets, the greater the likelihood of the eye seeing one uninterrupted & unmingled wavelength of one refracted output and
3. the larger the primary virtual facet area, the greater the opportunity for seeing one emitted uninterrupted refraction/dispersion event, as VF size decreases at some point the outputs become too small and low-energy for the eye to resolve, and they become white pinflashes

ETA: lighter gemstones that absorb less energy from the incident light have more energy to emit in their primary refractions, though the full relationship between frequency/absorption/dispersion/emission is one I've yet to see calculated

 

[Largosmom]

Yssie, layman's terms?

What is a virtual facet or virtual facet area?

I got that the larger the space from one facet to another (crown to culet?) the greater chance for dispersion to occur.

I think generally, the more facets, the more light dispersion, though some us prefer chunkier, fewer facets to lots of them. I think that this is a very difficult question as it is very dependent on the performance of a particular stone, it's inclusions, and the type of cut mure than the shape of the cut in terms of the way that the facets are arranged to give the most color and light performance in a colored stone.

 

[jleb]

In my experience you want a lot of facets on the bottom, a high crown and a lot of facets on the top. I cut a Madagascar sphene using a standard cushion cut and was very disappointed. I'm half way through re-cutting it and will get to it at some point. The point was made at the San Mateo gem show where I saw a sphene done just like I described. It was a kaleidescope of light exploding out of it. So I made a point of studying it very carefully. A Portuguese style cushion w/ high crown and a lot of crown facets would do the trick. The yield on round brilliants is around 18% of the rough volume, a Portuguese is about 23% of the rough volume. The cushions give you 28-43% return.

 

[AustenNut]

Y'all never cease to amaze me. Thank you very much, because I might be looking to have someone cut some material for me.

 

[Largosmom]

Also, the shape of the cut stone may be driven by the shape of the rough stone, which is why you see certain stones tending to be in certain shapes to maximize the carat weight.

 

[chrono]

Also, the shape of the cut stone may be driven by the shape of the rough stone, which is why you see certain stones tending to be in certain shapes to maximize the carat weight.

Ditto to Largosmom’s post; no cutter will want to waste more rough than necessary so will make the most of whatever rough material in hand by following the “natural” shape of the rough as much as possible.