Red spinel vs. red garnet?

[lovedogs]

If you are interested in Lab-Grown Ruby, I was at a shop earlier this week that was selling nice material for $30/ct at the Bangkok Jewelry Trade center.

I took a video of of this one because I thought it was cooI.

I didn’t pay attention to the name of the shop however.

If you or anyone else is interested (and it is permissible by PS rules), I can take a photo of their card and post it next time I visit there. Probably next week some time.

For the sake of transparency, I have no affiliation with the vendor. My interaction was all of 5 minutes long and I don’t even know their name.

It was just some stand right as you walk in the door.

They have lab-grown neon cobalt spinel and several other lab-grown gems for around the same price. I don’t know if they ship stones or not (though they probably do) but it might be a useful resource.

Yes please, post!

 

[SouthernGent]

T

If you are interested in Lab-Grown Ruby, I was at a shop earlier this week that was selling nice material for $30/ct at the Bangkok Jewelry Trade center.

I took a video of of this one because I thought it was cooI.

I didn’t pay attention to the name of the shop however.

If you or anyone else is interested (and it is permissible by PS rules), I can take a photo of their card and post it next time I visit there. Probably next week some time.

For the sake of transparency, I have no affiliation with the vendor. My interaction was all of 5 minutes long and I don’t even know their name.

It was just some stand right as you walk in the door.

They have lab-grown neon cobalt spinel and several other lab-grown gems for around the same price. I don’t know if they ship stones or not (though they probably do) but it might be a useful resource.

There's a guy relatively local to me who cuts some beautiful lab ruby. He also designs his own jewelry. His name is Phil Lagas-Rivera and goes by "alternativesatelier" on Instagram and Etsy.

 

[RRfromR]

Garnet. I've seen some Georgian foiled pieces and the colors are dreamy.

 

[SouthernGent]

Garnet. I've seen some Georgian foiled pieces and the colors are dreamy.

Do you have an example of what you're talking about?

 

[RRfromR]

Do you have an example of what you're talking about?

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[musicloveranthony]

Spinel is substantially harder and more durable
Spinel is profoundly more expensive
They both come in many many colors. Garnet has many different varieties with some having substantially more dispersion than spinel and even more than diamond. Personally, I find (good) garnet to be more sparkly than comparably colored spinel.


Three of the most eye-catching stones I have are garnets - a glowing orange spessartite garnet from Tanzania, a yellow grossular/andradite garnet from Mali, and a medium green grossular garnet from Tanzania. The yellow Mali garnet rivals any equally colored zircon or diamond I've seen for dispersion.

I have a few different shades of red and pink garnet and they don't look like spinel. It's a different mineral, so that makes sense. I wouldn't say either look better or worse - they just don't look the same. I think dollar for dollar garnet is going to be prettier than spinel - but it's not an apples to apples comparison

 

[Starstruck8]

Can anyone explain why spess and tsavs looks so awesome but the other garnets all look kinda meh?

Imagine a pink garnet with the Spess glow? It would look amazing. So why is it flat if they’re all garnets?

I doubt that it's any more (or less) mysterious than that corundum comes in spectacular blues and reds, good purples, yellows and oranges, but only so-so greens. (This is very noticeable if you look at a 'rainbow sapphire' tennis bracelet.) They are all corundum. As @Dr_Diesel says:

That's a great question! It must have something to do with the trace elements that account for the red color.

Here's my amateur theorizing. Glow depends on colour, inclusions (like the 'silk' of Kashmir sapphires), and fluorescence. Most (not all) high-quality gem garnets are clean and glassy and lack fluorescence, so that leaves only colour - it must be highly saturated. Saturation depends on the absorption spectrum, which depends on the chromophore ions and their local chemical environment. Note that some chromophore ions are part of the basic composition (e.g. spessartine by definition contains manganese, which gives an orangey colour), and some are impurities (e.g. chemically pure grossular would be colourless - the green of tsavorite is said to come from small amounts of chromium and vanadium). This site goes into great detail (fascinating, but probably more that you want to know...):
https://www.gemstonemagnetism.com/template

The catch with all this is that it's handwaving, not actual explanation. There has to be a reason, in terms of the absorption spectra, why typical almandine garnets have good flashes, but are dark and tend to brown, typical red spinels have a 'watery' look, and good rubies look 'intense'. But I haven't come across any such account. If anyone has, I'd be pleased to hear.

The closest I've seen is this, but it's for corundum:
https://www.gia.edu/doc/sp20-corundum-chromophores.pdf
Note that it shows the spectra and the resulting colours, but it doesn't directly address glow. Something like that but for garnets would be interesting.

 

[Dr_Diesel]

I doubt that it's any more (or less) mysterious than that corundum comes in spectacular blues and reds, good purples, yellows and oranges, but only so-so greens. (This is very noticeable if you look at a 'rainbow sapphire' tennis bracelet.) They are all corundum. As @Dr_Diesel says:


Here's my amateur theorizing. Glow depends on colour, inclusions (like the 'silk' of Kashmir sapphires), and fluorescence. Most (not all) high-quality gem garnets are clean and glassy and lack fluorescence, so that leaves only colour - it must be highly saturated. Saturation depends on the absorption spectrum, which depends on the chromophore ions and their local chemical environment. Note that some chromophore ions are part of the basic composition (e.g. spessartine by definition contains manganese, which gives an orangey colour), and some are impurities (e.g. chemically pure grossular would be colourless - the green of tsavorite is said to come from small amounts of chromium and vanadium). This site goes into great detail (fascinating, but probably more that you want to know...):
https://www.gemstonemagnetism.com/template

The catch with all this is that it's handwaving, not actual explanation. There has to be a reason, in terms of the absorption spectra, why typical almandine garnets have good flashes, but are dark and tend to brown, typical red spinels have a 'watery' look, and good rubies look 'intense'. But I haven't come across any such account. If anyone has, I'd be pleased to hear.

The closest I've seen is this, but it's for corundum:
https://www.gia.edu/doc/sp20-corundum-chromophores.pdf
Note that it shows the spectra and the resulting colours, but it doesn't directly address glow. Something like that but for garnets would be interesting.

I think the appearance of a "glowing" stone (particularly spressartite, garnet and spinel) often co-occurs with nice crystal, vivid saturation (as you mentioned), and medium tone.

Most red garnets have a dark tone. There are definitely some over-dark tsavorite and spessartite out there that also seem to lack that "glowing" quality.

Does anybody know what influences tone?

It seems like there is *some* correlation between saturation and tone, but there are definitely stones with "meh" saturation and over-dark tone. I'm guessing that this suggests some other factor is involved.

In ruby and spinel, that factor is typically iron.

Maybe the fact that iron accounts for the red color in garnets accounts for the dark tone? I really don't know...I'm just extrapolating.

 

[Starstruck8]

I think the appearance of a "glowing" stone (particularly spressartite, garnet and spinel) often co-occurs with nice crystal, vivid saturation (as you mentioned), and medium tone.

Most red garnets have a dark tone. There are definitely some over-dark tsavorite and spessartite out there that also seem to lack that "glowing" quality.

Thank you, @Dr_Diesel. I was hoping someone would take this up. Glow is a big part of the magic of gemstones, but I have never found a good account of what exactly causes it.

Here's my pet theory. Warning: I haven't seen it explicitly seen it stated like this, so it may be way off. Much of the magic of faceted stones is that a facet can look brighter than is physically possible for ordinary diffusely reflecting objects of the same colour. For colourless stones like diamonds, this excess brightness is seen as brilliance. For coloured stones, it's seen as glow. (This is also why faceted stones are hard to photograph. If the background is properly exposed, the brighter facets will be burned out.)

This happens because stones are cut to catch light from behind the viewer, but at not too wide an angle. The light there is often brighter than the average from the whole environment. Facets can also catch reflections from small light sources.

Here is a demo: Put a stone in a light box that gives very even light. The flash and glow disappear, and you see only the basic colour. Similarly, colourless stones just look white, and polished metal loses its highlights and looks flat. This amazed me when I first saw it. It also recalls the standard advice that you should judge the colour of a stone in soft light.

To illustrate the idea: For diffusely reflecting objects, the possible colours for a given hue (one hue sheet of a Munsell or similar colour tree), are, very schematically, something like the left figure below. Light tone and dark tone both imply low chroma. High chroma is possible only somewhere around mid tone. If a facet of a stone with true colour A (right figure) is catching brighter than average light, the light coming to your eye may be like B. This is outside the normal range. Your visual system will interpret this as glow or brilliance. But this can happen only if the true colour is not too far from the edge of the range of possible colours. See the shaded area in the right figure. (How wide this area is will depend on how much the lighting varies).



I like this story. It explains a lot. Why jewellery shops like multi-point light. Why light filtered through trees is good for viewing diamonds. Why a light box kills glow. Why stones that are too dark don't usually glow. (The right figure suggests that glow is possible for dark stones, but only in a narrow band of high saturation.) But all that doesn't mean the story is right. Comments welcome.

 

[CBianco]

So my theory is that it simply has something to do with the rough material itself.

I'll give an example with a precision cut pink garnet (top round) and a pink spinel (bottom cushion)


The round in ring is a mahenge spinel.
Gemstones are faceted to get the best look face up, if cut properly of course.
So as you can see when the lighting is good meaning that the facets are light up, the pink garnet looks similar to spinel because of its great cutting that maximizes all the potential of the material. However when the lighting is not ideal and the facets are off, all you can see is the body color of the material. In this case both spinel's body color is much more saturated than the garnet, and I don't think I've ever seen a bright and neon pink garnet in rough form. If you google jedi spinels, you can see that even when rough they are neon. When these stones are set in rings and we wear them in imperfect everyday lighting conditions, the difference is quite big. There's a reason why garnets are a couple hundred per ct. at best and some neon spinels are getting to the way overpriced 5 digits per ct territory, and that includes rarity as well.

 

[SouthernGent]

So my theory is that it simply has something to do with the rough material itself.

I'll give an example with a precision cut pink garnet (top round) and a pink spinel (bottom cushion)


The round in ring is a mahenge spinel.
Gemstones are faceted to get the best look face up, if cut properly of course.
So as you can see when the lighting is good meaning that the facets are light up, the pink garnet looks similar to spinel because of its great cutting that maximizes all the potential of the material. However when the lighting is not ideal and the facets are off, all you can see is the body color of the material. In this case both spinel's body color is much more saturated than the garnet, and I don't think I've ever seen a bright and neon pink garnet in rough form. If you google jedi spinels, you can see that even when rough they are neon. When these stones are set in rings and we wear them in imperfect everyday lighting conditions, the difference is quite big. There's a reason why garnets are a couple hundred per ct. at best and some neon spinels are getting to the way overpriced 5 digits per ct territory, and that includes rarity as well.

This was very helpful - thanks for sharing.
What is the middle stone? It doesn't seem to be affected as much by the lighting.

 

[Starstruck8]

On glow, this picture is worth a thousand words of explanation:


Yes, the orange/brown patches really are the same. I checked the RGB values.

Our visual system interprets the picture a 3D scene, takes the 'white' squares as white, and uses them as a reference to judge the local illumination. The patch on the shadowed 'white' square is lighter than its reference square, so it seems to glow.

This illustrates the way facets that can flash good orange (or red) when they catch the light can brown out when they don't, or in softer light.

 

[CBianco]

This was very helpful - thanks for sharing.
What is the middle stone? It doesn't seem to be affected as much by the lighting.

Bi-color sapphire! It was the coolest stone

 

[SouthernGent]

Bi-color sapphire! It was the coolest stone

I agree!

 

[Dr_Diesel]

So my theory is that it simply has something to do with the rough material itself.

I'll give an example with a precision cut pink garnet (top round) and a pink spinel (bottom cushion)


The round in ring is a mahenge spinel.
Gemstones are faceted to get the best look face up, if cut properly of course.
So as you can see when the lighting is good meaning that the facets are light up, the pink garnet looks similar to spinel because of its great cutting that maximizes all the potential of the material. However when the lighting is not ideal and the facets are off, all you can see is the body color of the material. In this case both spinel's body color is much more saturated than the garnet, and I don't think I've ever seen a bright and neon pink garnet in rough form. If you google jedi spinels, you can see that even when rough they are neon. When these stones are set in rings and we wear them in imperfect everyday lighting conditions, the difference is quite big. There's a reason why garnets are a couple hundred per ct. at best and some neon spinels are getting to the way overpriced 5 digits per ct territory, and that includes rarity as well.

Here’s another example of glowy material: the backside of a 6ct Afghani Rubilite. This batch is crazy neon. It looks like Mahenge spinel without the fluorescence.



@starstruck,
I think you’re on something about the reflection that we have from facets and contrast.

I’ve seen many stones that seem to glow in diffuse lighting, But in direct lighting, some of the facets appear overly bright and the surrounding facets appear overly dark.

I guess, once again, this speaks to both the importance of material and cutting.

Whereas colorless diamonds are usually cut for a light return, colored diamonds are usually cut for color retention.

A lot of native cut colored stones are quite good at the latter.

Sometimes, modern cuts with colored stones can be a bit disappointing because they create too much contrast.

While they are more flashy, that can also sabotage the opportunity for the material to exhibit its innate glow.

Garnets are actually notorious for this.

I once had a beautiful fuchsia, Mozambique Garnet that had a slight window. I had it re-cut to eliminate the window, and as soon as the culet was completely closed, the whole thing went super dark.

I asked lapidary about it and he said it’s actually a well-known phenomenon in Garnet.

I kind of wish he would’ve told me beforehand. I seriously regret recutting that stone. It totally killed the openness of the color.

This is the Garnet:



Here’s an example of where it happened with a Spinel as well:

 

[Dr_Diesel]

Here’s another example of glowy material: the backside of a 6ct Afghani Rubilite. This batch is crazy neon. It looks like Mahenge spinel without the fluorescence.



@starstruck,
I think you’re on something about the reflection that we have from facets and contrast.

I’ve seen many stones that seem to glow in diffuse lighting, But in direct lighting, some of the facets appear overly bright and the surrounding facets appear overly dark.

I guess, once again, this speaks to both the importance of material and cutting.

Whereas colorless diamonds are usually cut for a light return, colored diamonds are usually cut for color retention.

A lot of native cut colored stones are quite good at the latter.

Sometimes, modern cuts with colored stones can be a bit disappointing because they create too much contrast.

While they are more flashy, that can also sabotage the opportunity for the material to exhibit its innate glow.

Garnets are actually notorious for this.

I once had a beautiful fuchsia, Mozambique Garnet that had a slight window. I had it re-cut to eliminate the window, and as soon as the culet was completely closed, the whole thing went super dark.

I asked lapidary about it and he said it’s actually a well-known phenomenon in Garnet.

I kind of wish he would’ve told me beforehand. I seriously regret recutting that stone. It totally killed the openness of the color.

This is the Garnet:



Here’s an example of where it happened with a Spinel as well:

I’ve always hated windows and colored stones, but I’m starting to learn that there may be some wisdom behind it - at least in some cases.

… It’s not unlike modern diamonds versus antique diamonds.

Sometimes the old Masters knew what they were doing better than the modern cutters of today.

 

[Starstruck8]

I'll give an example with a precision cut pink garnet (top round) and a pink spinel (bottom cushion)

The round in ring is a mahenge spinel.

Off topic, but your photographs are wonderful. In the first picture you have captured the 'on' facets without clipping. (Except for a very few facets of the spinel.) That's quite a feat! How did you do it? Just the right balance of directionality and softness in the lighting? Tricky processing? A camera with defaults that just work? I'd really like to know.

Do the 'on' facets of the spinel look pink (as in the photo) in real life, or do they look more red as the body colour and the second pic suggest? I ask because for my red spinel, the facets always look more or less red, but sometimes photograph pink (due to clipping). I'm suspecting that your camera or processing may have some sort of highlight roll-off that's reducing the apparent difference between the spinel and the garnet.

So my theory is that it simply has something to do with the rough material itself.

Yes, and that something is surely the absorption spectrum (if you leave aside fluorescence). The tricky bit is to explain precisely how the absorption spectrum influences glow, and why red spinels typically have 'better' spectra than red garnets. I've had trouble finding spectra that I can confidently interpret. This GIA page links information of the stones of their Gueblin collection - it includes photos, spectra and some chemistry, which may be useful to someone who knows what to make of it.
https://www.gia.edu/gia-gubelin-gem-project-research
I've made some attempts, but I'm not ready to post them.

I’ve seen many stones that seem to glow in diffuse lighting, But in direct lighting, some of the facets appear overly bright and the surrounding facets appear overly dark.

This is my experience too. Direct sun is not good, because the 'on' facets dazzle the eye and make the 'off' facets look dark by contrast. Light box softness is not good, because it removes all contrast. What you want is Goldilocks style 'just right'.

 

[CBianco]

Off topic, but your photographs are wonderful. In the first picture you have captured the 'on' facets without clipping. (Except for a very few facets of the spinel.) That's quite a feat! How did you do it? Just the right balance of directionality and softness in the lighting? Tricky processing? A camera with defaults that just work? I'd really like to know.

Do the 'on' facets of the spinel look pink (as in the photo) in real life, or do they look more red as the body colour and the second pic suggest? I ask because for my red spinel, the facets always look more or less red, but sometimes photograph pink (due to clipping). I'm suspecting that your camera or processing may have some sort of highlight roll-off that's reducing the apparent difference between the spinel and the garnet.


Yes, and that something is surely the absorption spectrum (if you leave aside fluorescence). The tricky bit is to explain precisely how the absorption spectrum influences glow, and why red spinels typically have 'better' spectra than red garnets. I've had trouble finding spectra that I can confidently interpret. This GIA page links information of the stones of their Gueblin collection - it includes photos, spectra and some chemistry, which may be useful to someone who knows what to make of it.
https://www.gia.edu/gia-gubelin-gem-project-research
I've made some attempts, but I'm not ready to post them.


This is my experience too. Direct sun is not good, because the 'on' facets dazzle the eye and make the 'off' facets look dark by contrast. Light box softness is not good, because it removes all contrast. What you want is Goldilocks style 'just right'.

Thank you! I'm far from a photography expert lol, and use a Samsung S23. I just happened to be in a good spot in my yard where the diffused lighting was perfect. The cushion pink spinel is actually slightly less purplish than the picture in person, but not red. I don't have the garnet with me anymore so I can't recall exactly what the difference was, but that they were similar with the spinel being a little more saturated.


My phone camera can make redder gems look too pink as well, it seems like it's a common problem. I've found that photos and even videos of gems never look exactly the same as in person.

 

[Starstruck8]

Thank you! I'm far from a photography expert lol, and use a Samsung S23. I just happened to be in a good spot in my yard where the diffused lighting was perfect. The cushion pink spinel is actually slightly less purplish than the picture in person, but not red. I don't have the garnet with me anymore so I can't recall exactly what the difference was, but that they were similar with the spinel being a little more saturated.


My phone camera can make redder gems look too pink as well, it seems like it's a common problem. I've found that photos and even videos of gems never look exactly the same as in person.

Thank you! You are too modest. You have the crucial things: the mind to know what to look for and the eye to see when you have achieved it.

 

[SouthernGent]

Thank you! You are too modest. You have the crucial things: the mind to know what to look for and the eye to see when you have achieved it.

What is the blue stone?

 

[CBianco]

What is the blue stone?

It's a blue zircon

 

[Starstruck8]

So my theory is that it simply has something to do with the rough material itself.

Most red garnets have a dark tone. There are definitely some over-dark tsavorite and spessartite out there that also seem to lack that "glowing" quality.

Does anybody know what influences tone?

It seems like there is *some* correlation between saturation and tone, but there are definitely stones with "meh" saturation and over-dark tone. I'm guessing that this suggests some other factor is involved.

So I looked at the spectra in the GIA Guebelin project for this garnet (collection no. 35213) and this spinel (collection no. 35392). This chart shows the chroma and lightness calculated for varying lengths of light path in each:


Warning: I'm an amateur playing with tools I don't fully understand... The calculations were done in the CIE L*a*b*colour space with D65 illuminant. Lightness (tone) is CIE L* and Chroma (strength of colour) is CIE C* (= square root of a* squared plus b* squared). There is no allowance for surface reflections or scattering.

The top of each curve represents zero path length, so no absorption, so white . The bottom represents infinite path length, so complete absorption, so black. It's only in the mid tones that there is strong colour. The circles show the calculated colour at highest chroma. Note that these are the colours you would see in a light box with even lighting; you would expect to see lighter and stronger colours in the 'on' facets in typical uneven lighting.

The spinel always gives stronger colour than the garnet at same lightness. The absorption spectra show why:


(Note: Absorbance is logarithmic: Zero = no absorption, full transmission. 1 = 10% of incoming light is transmitted, 2 = 1% of incoming is transmitted, etc.
Transmission is linear: Zero = none of incoming light is transmitted, 0.1 = 10% of incoming light is transmitted, etc.)
(How can these curves result in about the same lightness, when the transmission curve for the spinel looks so much higher? Answer: because CIE L*, which is intended to represent something like perceived lightness, depends mostly on the wavelengths around 550nm, in which the garnet transmission curve is higher. The high transmission of the spinel in the long wavelengths make it look redder, but doesn't contribute much to the lightness.)

So, TL; DR: Why does the spinel have stronger red than the garnet? Both are reddish because the absorbance for long wavelengths (above 600nm) is lower than that for shorter wavelengths. But for spinel, it's much lower, so the spinel looks redder.

And why are the spectra like that? I'll leave that one to someone else. I'd guess that it has to do with chromium as the key chromophore in red spinel and iron as a key chromophore in red garnets. But that's just a guess. Even if it's true, it's not an explanation.

Usual warnings: amateur at work, fools rush in, shortcut assumptions etc. I've looked at only two stones. To do more would be too much like hard work. And I'd be surprised if something like this hadn't already been done by someone better qualified.