Half and half

[colormyworld]

I have been seeing this term used a lot more around the forum lately. Almost like a trendy new phrase. I was under the impression that this is an effect seen in ovals and elongated cushions when the single point light is in line( or approximately) with the gem then the eye. The side of the gem closest to the eye will show brilliance while the side closest to the light source is not showing any. Perhaps some of the gem cutters and experts could shed some light on the subject for us.

 

[Harriet]

I thought it referred to what I have with my tea. In all seriousness, I don't understand what it means when applied to gems.

 

[Jim Rentfrow]

I think what colormyworld is refering to is when one side of a gemstone is very brilliant and the other is basically dead. One side will be returning 70-80% of the light to your eye while the other is returning 30-40%. It can be common in ovals but I have not seen it in cushions or other shapes.

 

[movie zombie]

turn the stone and it disappears....unlike the bowtie effect which remains no matter which way the stone is turned. i can live with it for the right color at the right price.

MoZo

 

[Harriet]

Jim,
Thanks for the explanation. I honestly have not seen that phenomenon.

 

[MakingTheGrade]

Here's a link to a previous thread I had started on the subject after buying an oval spinel.
Gene and Michael and many other experts were sweet enough to chime in.
https://www.pricescope.com/community/threads/oval-cuts-half-n-half-extinction-effect.116619/

 

[LD]

Here's a link to a previous thread I had started on the subject after buying an oval spinel.
Gene and Michael and many other experts were sweet enough to chime in.
https://www.pricescope.com/community/threads/oval-cuts-half-n-half-extinction-effect.116619/

I'm surprised that the term half/half is thought of as "trendy" when quite clearly it's a descriptor of a particular result of a cut that was discussed in detail on the link kindly provided by MTG and is not (generally) something that people want in their gems.

 

[MakingTheGrade]

Oh, I think they just meant that the phrase "half and half" is trendy, not the phenomenon itself. Haha, hope I didn't inadvertently coin that term, it doesn't exactly roll of the tongue or sound particularly scientific.

 

[chrono]

I call it half and half because it is literally as though the stone is cut in half; half is bright and half is dark. Of course, when I first used it as a descriptor, I was worried other might think I’m talking about coffee creamer.

 

[LD]

It's a very apt descriptor and used by many.

 

[Michael_E]

I have been seeing this term used a lot more around the forum lately. Almost like a trendy new phrase. I was under the impression that this is an effect seen in ovals and elongated cushions when the single point light is in line( or approximately) with the gem then the eye. The side of the gem closest to the eye will show brilliance while the side closest to the light source is not showing any. Perhaps some of the gem cutters and experts could shed some light on the subject for us.

Unfortunately this term, along with "tilt windowing" is a bit misunderstood and get's tied to the idea that a stone is poorly cut, when the reality is that any stone will show this effect in the right, (or maybe it's the wrong), lighting. When light enters a stone from the side, it crosses the stone and bounces up at your eyes from the side opposite where it entered. Likewise if the side opposite the light is dark, then that darkness gets reflected from the other side of the gem.

In some gems there is enough light bouncing around in different locations that the stone never shows a distinct line between dark and light sides. Stones which have micro inclusions or strong fluorescence "glow" no matter where the light is coming from since some of the light which is crossing the stone, gets bumped out in the direction of your eyes from every place in the stone. Other stones which have high R.I. and are not too dark in tone will have enough light bouncing back across the center of the stone and out from the "dark side" that they don't show this as obviously as other stones. The stones in which this is most evident are clear stones, having low R.I., perhaps a bit on the dark side already and being elongated. On the elongated stones, the effect is worse in the long direction simply because the length of the stone makes the bounce back angle smaller and so less light can bounce back to come out the dark side. This is particularly bad in darker garnets, since any light which does bounce back gets absorbed by the dark stone and never makes it out to your eyes.

How do you avoid it. Buy low aspect ratio stones which have a high R.I., micro inclusions or fluorescence and vivid, saturated color that it not so dark that light gets "eaten" as it crosses the stone. In essence, but expensive, high quality gems with good to excellent cutting. Cutting by the way has little to do with this effect and so it is NOT a mark of poor quality cutting as some have implied here previously, Like tilt windowing, it's just a built in effect that depends on the, lighting conditions, material and aspect ratio of the stone. I hope that helps!

 

[MakingTheGrade]

In my experience, I have never seen this effect in supernova ovals.
Is there an explanation as to why supernova cuts seem to be spared?

 

[ooo~Shiney!]

Is this what is meant?
I love the fact that my stone has soooo many personalities!

 

[movie zombie]

bingo, #3 is the winner for half and half. but its not always there and if you turn the ring around, i bet its now on the far side from you.

a very different animal than extinction......yuck.

MoZo

 

[Pandora II]

My e-ring tsavorite does it in certain lights (in fact I think you can see it in my avatar photo). It's an elongated cushion.

For a while I was a bit worried that it was the 'dreaded' extinction and how could this be given the provenance and cutter of the stone , then I realised that a) I read far too many anally-retentive PS threads and b)even though it does this, the darker half still shows green it just looks 'half and half' - turn the stone 90 degrees and it vanishes. Other lights - especially overcast daylight - you can't see it at all or only faintly.

 

[colormyworld]

Thank you Micheal.

Pandora +1 to a

 

[Arkteia]

Is this what is meant?
I love the fact that my stone has soooo many personalities!

I still think your stone has a large bowtie, not really a half and half. I love its color and the setting. Who set it? The ring is simple and elegant.

 

[ooo~Shiney!]

Is this what is meant?
I love the fact that my stone has soooo many personalities!

I still think your stone has a large bowtie, not really a half and half. I love its color and the setting. Who set it? The ring is simple and elegant.

Hey crasru, it was one of my first CS purchases and it's from Swala.
I had it set by Sally.
It's kind of a delicate setting for my hand, but I love the stone and I love to wear it!
It is always looking different, which I find charming

 

[chrono]

I have seen this half and half on precision cut stones as well so I know it’s not a function of a poor cut, but rather the wrong choice of design. I don’t think just any stone will show this effect in the “right” lighting. I have elongated rectangular cushions and ovals that do not do this under any lighting conditions. I have also personally handled precision cut ovals and elongated rectangular cushions that do this and they went back to the vendor right away. A friend of mine had a discussion about this with a precision lapidary and he said that certain designs on certain gem varieties are more prone to this. I think he said that if such a stone had a keel rather than a pointed culet, this can be avoided. I agree with Michael that I tend to not see this when:
1. Stones have fluorescence or micro inclusions
2. Stones have high RI and not overly dark.

I see this more often in tourmalines where they tend to be cut in elongated shapes, have low RI, can be very dark or very light.


Shiney,
That’s the half and half extinction. Thankfully, you won’t see it all the time. Colour looks hot and well saturated.

 

[Arcadian]

I tend to have only seen this in ovals that I have, not in any other stone.

If I'm bugged by it unset, then its just a stone I won't set. It doesn't make it any less beautiful at least not to me, but to have to cock your head anytime you want to see the full effect of your stone (especially while driving, yes I've done this so don't do this kiddies) can be a bit of a drag.

But yes I do have some set stones that will do the half and half thing occasionally. I'm cool with it as long as it doesn't do it in every lighting situation.


-A

 

[LD]

This is a precision cut garnet and the half/half can clearly be seen in some lights. In others, it's not present at all. This is a fascinating gemstone and I actually like the way it can look totally different at both ends.

 

[movie zombie]

Loving, your half and half stone wouldn't bother me at all. very nice stone.

Mozo

 

[LD]

Loving, your half and half stone wouldn't bother me at all. very nice stone.

Mozo

Thank you Mozo. It doesn't bother me in the slightest! In fact I''m fascinated with it. Interestingly, somebody asked opinions on 4 stones recently and one looked similar and I wondered if it would have the half/half chameleon tendancies as well.

 

[Arkteia]

Chrono, that explains. If Shiney's stone is half and half, then my spinel from Swala is even more so. I have other stones, beautiful and precious, in which the effect is much more pronounced than in Shiney's ring. Certain bright stones, such as chrysoberyls and spinel, can still stand it, others do not look well at all.