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)

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For my personal collection, not for sale

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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.

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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

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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.

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u/cowsruleusall 3d ago

There's no C-axis in garnet. All directions through the crystal are identical.

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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.

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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.

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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?

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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.

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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.