Question - what causes dark rim on faceted stones?

[Desertrose]

I'm wondering why some cut stones show this darker rim on around their edges...does that indicate a bad cut?

Here are example pics (and I don't mean to pick on any particular cutter, I see these frequently):



Same stone different lighting, just to show it's not the lighting causing the rim:



I see this issue often on colored melee. Very apparent when you place the stones table down, some will have the rim and some less or not at all. How does this affect light performance?

 

[Barrett]

Some issue with the girdle? To thick or to thin?
To me it looks like the view point of the camera isn't centered with the stone, which then mucks up the light rays bouncing back out of it.
Do you see it with only this stone, certain stones, or....?
"Ring-around-the-collar...err...gemstone"

 

[Desertrose]

Lol, "ring around the collar" - exactly!!

It's dark here so I can't take a good photo of my melee, but here is one I found online that shows the dark rim on some of the stones. I see it when looking straight down on stones, not just at an angle, so I don't think it's about perspective...



It seems like it might be too thin of a girdle, but I don't know why that would cause darkening.

 

[shkra11]

Im very new to all this, but that immediately reminded me of some of the signs of certain treatments. There are some stones that look similar on page 7 of this article due to diffusion treatments: https://www.gia.edu/cs/Satellite?bl...entFile&blobwhere=1355958396200&ssbinary=true

 

[chrono]

Spessartite is untreated so that is definitely not the cause. I have only seen this "ring" in a few pictures but have not come across any in person (both precision and non-precision cut stones). I hope our usual lapidaries will have some free time to pop in to help answer your question.

 

[adastra]

I am new here so I am not qualified to answer. But confidentially I come across Richard Hughes's article talking about extinction: http://www.ruby-sapphire.com/brilliance_windows_extinction.htm

"Generally, the pavilion facets closest to the girdle are cut too steep while those at the culet are too shallow; often only those in between are cut at the proper angle. This results in three distinct zones: extinction near girdle, windows near the culet and brilliance in-between, as shown clearly in the step-cut pavilion below." (very bottom of the page, Fig. 5)

Not sure if the above is relevant but just throw in to see if that helps the discussion.

yes, and I agree with Chrono that lapidaries will be in a better position to tell the reason(s).

 

[qubitasaurus]

I am wondering if an argument made based on cirtical angles and total internal refraction would still predict the dark colour at the rim when the stone is flipped upside down (i.e. light entering from the pavilion fascet and strinking the table rather than the other way arround). I actually have no idea, but flipping it upside down must essentialy change all the angles at which the light is incident on the first surface it scatters off, so I am curious. If the effect is often observed in melee, but not neccesarily seen in larger stones of the same type, then I suppose it is most likely to be the cutting. It would be nice to take a parcle with a few melee that dont exhibit the effect and some that do and try to decide if anything looked different. Also it is a bit of a crazy idea but maybe if you put the stuff in water the critical angles will all change, and it would be interesting to see what happened to the dark rims. Lol that sounds a little crazy but if I got bored I would probably try it anyway.

 

[normanintheskywithsapphires]

Isn't that simply because there is no light-reflection in the outer edge? The 'inside' is cut to throw light back and thats what we see, only in the rim there is no mirror effect so it looks darker but in fact it is only not reflecting?

 

[Desertrose]

Thank you adastra for that excellent article! What a useful read for anyone into gemstones. That helped me to understand better the RI and cutting and how they relate, thanks!

Quibitasaurus, I think you are right, flipping stones does seem to make a difference. I have taken two photos of some hauynite melee that have the darker rim. The degree varies from stone to stone. The dark rims seem more pronounced with the stones upside down, which makes sense from what I learned from adastra's article - they are cut to reflect light back up, which would cover up any extinction or this dark rim "shadowing".

First picture is looking down on the stones, flipped upside down, in their box lid. There is a bit of shadow from box, but you can still see the rings:



Here are the same stones, right side up, in their box:



Some dark rim is still visible when the stones face up (for example the bottom left corner stone), but it is much less pronounced.

To me, it almost seems like the thin area of the girdle makes the color more dense? Maybe that is the "dark rim" effect?

 

[qubitasaurus]

I don't know, the only options I see are to ask some cutters, or alternatively to try to play with a few stones to try to isolate out what the most probable explanation is. I would love to play, but I have systematically set the small number of stones I have bought. Obviously I need to buy more , I wonder if my husband will agree with this argument?

If I was able to play I would start with the blind guess that the gem is deliberately cut so that most of the light entering from the table bounces off the pavilion facets and gets reflected back up through the table, when the stone is face up. Thus when you flip it upside down the light entering the pavilion facets tends to have a direct path down to the table, causing it to be more or less see through -- so it is not so much that there is a rim, but rather that there is a large transparent part in the center. I have no idea what I am talking about , so I would probably go try to disprove this guess by looking at lots of big and small (pale color) stones. Trying to see if even the big ones have a thin dark rim when I flip them, and then start looking at different cuts. Then if i managed to disprove it, I would start again with a new guess and hopefully a better idea from all the stones I flipped upside down and left finger prints all over.... There might be a bunch of different reasons, maybe some of them will just be cut poorly like adastra suggested. Or maybe that is really what is going on all the time . Would love to play -- edited to say most of this might just be crazy so feel free to ignore it if you like.