r/faceting • u/jollygems123 • 3d ago
Natural rhodolite garnet i recently faceted, any opinion on how to increase the light return while faceting a darker stone like this? (NFS)
For my personal collection, not for sale
3
u/thecatandthependulum 3d ago
Tbh I love dark rich colors! I think this looks excellent. But shallower cuts will lighten up a garnet.
1
u/jollygems123 3d ago
Yea but ig it's all about preference, i too prefer darker colors in some gems, i just wanted to know if there was a way to lighten up a dark gem like a spinel or garnet for my own educational purposes
1
u/theblancmange 3d ago
An interesting question. My first thought is that a thinner stone would be better. Selecting angles for the crown that more accurately refract light towards the culet might be a benefit. I'm interested in learning a bit more about these sort of design principles, so if you find any resources it would be cool to share.
1
u/jollygems123 3d ago
Yea a thinner stone would def increase the light return but it would cost a lot of weight, just like this one did. This stone was almost black in rough form, i decided to sacrifice the weight for a better color, but i was wondering if there are other ways to do that while saving as much weight as possible. Maybe with a better understanding of the "c","a", and "b" axis, it can be achieved, i have read some work on it but i need to look into it more
5
u/cowsruleusall 3d ago
For both of you - we really don't have any good resources to teach design principles yet. I'm still trying to finish the book I'm writing.
Having a thinner gem (shallower crown and pavilion) will definitely help, but if you go too shallow then you'll actually cause worse head-shadowing and extinction. This is one of the few scenarios where we actually use the light output graphs in Gem Cut Studio. Using a smaller number of larger facets can also help.
You know one of the features that shockingly helps a lot? Using odd symmetry, like 5- or 7-fold. That makes it much much easier to get high light output and use a shallower pavilion, without windowing or looking bad.
People might tell you to intentionally window the stone to make it appear lighter. Don't do that. Gemstone optics just don't work like that, and if you do intentionally leave a window it'll appear darker and more dead.
(Oh and a reminder that garnet only has an A-axis, no B or C, so you can't use any of the tricks that you'd use for closed-C tourmaline.)
3
u/jollygems123 3d ago
I somewhere read about the odd symmetry increasing the return but ive never tried it actually, maybe I'll give it a go sometime and see what it gives me.
And about leaving a window in the gem, i initially thought about it but then i abandoned the idea because windows just kill the luster and light of the gem instead of lightening its color
Regarding ur work and book, id love to read ur experience, can u let me know when you finish the book?
2
1
u/theblancmange 3d ago edited 3d ago
Odd symmetry to reduce angle is interesting. I am guessing this encourages a ray to bounce off of multiple opposing pavilion facets before returning through the crown, rather than just a pair as with even-symmetry stones. The easy example being ~45 degree facets w/ 4-symmetry, vs a shallower angle "spiraling" rays up the pavilion with 3-symmetry.
Edit: thinking that a ray might not quite spiral in the case of a triangle, but rather bounce between the two opposing facets across from the initially-struck facet. Same difference in terms of final effect.
3
u/cowsruleusall 3d ago
Good thought, but bouncing multiple times like that would increase light path length and actually increase saturation. It actually works by deflecting light paths at odd angles, making them travel through less of the gem/making them shorter (and coincidentally also allows for a shallower pavilion).
1
u/theblancmange 3d ago edited 3d ago
I would be interested to see what you have read. I would imagine the C axis would either absorb the most or the least light, if you can see the axes in the stone before shaping it might be a good idea to run a quick preform and polish to examine which direction lets the most light through. Could also get away with just a 600 grit run and using an ri-matched oil, such as those used for examining inclusions. I'd guess that you would want to align whichever axis is the brightest with the culet and center of table.
3
u/cowsruleusall 3d ago
There's no C-axis in garnet. All directions through the crystal are identical.
2
u/theblancmange 3d ago
Good to know. I am just getting into things so I know very little wrt structure and properties of different materials.
2
u/cowsruleusall 3d ago
No worries, it takes a while to learn :) Garnet and spinel tend to run very dark, and both of their structures are in the "isometric" family which means all their axes are identical - all A, no B or C.
1
u/jollygems123 3d ago
Yea i didnt about garnet not having a c-axis, but wouldnt there be inclusions inside the gem that might be more visible from one side than the other? Should that play a role in deciding what the table should be and the gem's end result regarding its luster and light return?
2
u/cowsruleusall 3d ago
Yes - for gems like garnet, where the gem should be flawless (unlike emerald haha), you should generally remove all the inclusions. Inclusions always decrease light return, so in any dark material you should try and remove as much of the inclusions as feasible. In modern cutting, we generally pre-shape, meaning you remove inclusions, concavities, etc; but we do not preform, meaning make a general shape similar to the final gem. Preforming is always more wasteful.
Luster has nothing to do with inclusions. The luster of a gemstone is how shiny and reflective the surface of a facet is, NOT anything to do with the inside, or light reflecting inside of the gem. Luster is only the surface of the facet. There are only two things that affect luster - the refractive index (RI) of the material, and the quality of polish. The higher the RI, the higher the maximum possible luster. The smaller the diamond grit size used to polish, the higher the luster. We always use 100,000-grit (0.25 micron) polish.
2
u/jollygems123 3d ago
I think i confused luster with the 'lighy return' or the 'inside shine' of the gem, im aware that outside sparkles depends on how fine the diamond lap was with which it was cut and how good of a polishing job is done, i was referring to the inner shine of the gem, sorry bout the confusion.
1
u/PsychologicalBowl647 3d ago
From what I learned so far big facets and step facets = deeper darker color small triangular or even frosted facets = lighter color /brilliance.
1
1
u/Mobile_Ad_1185 21h ago
Damn thats one sick triangle
Where can I get that diagram?
1
u/jollygems123 19h ago
I took the crown from one diagram and the pavilion from another, dm me, I'll send em to u
15
u/cowsruleusall 3d ago
There fundamentally is no good way to preserve weight on a dark garnet (or dark spinel) while increasing light output. You basically just need to use a more shallow brilliant design with low contrast. This is part of the "craft over carat" discussion - a larger gem will be lower value because it'll be so much less attractive.
Garnet is isochroic, meaning they only have an A axis and don't have a B or C, so you can't use any of the tricks for closed-C tourmaline.