Question about metallic luster in gemstones

[CaseyLouLou]

I’m trying to figure out why some gemstones have a metallic luster and some are just sparkly without that metallic look. Is it the cut or the color or the undertone? It seems to happen in pale pink spinels and darker pink ones sometimes and in sapphires that are medium blue but not all of them do this so I’m confused.

Any insights into this phenomenon? Thanks!

 

[Bron357]

My guess is that it’s the presence of fine crystalline inclusions.
Faceting achieves sparkle by angled light return to the eye whereas crystalline inclusions will still reflect and refract light but in a haphazard way, dependent on their presence and position within the gemstone.
My wee Alexandrite throws tiny rainbow sparkles, it’s when light, at the right angle, creates a prism on the crystal inclusions.

 

[Starstruck8]

This one has me puzzled too. I'm pretty sure colour is involved. To my eyes, unsaturated spinels usually look metallic, but strongly saturated red and blue ones don't. Aquas are similar: the pale blue ones look 'steely', the stronger darker blue ones don't. Note that paraibas, with a similar hue to pale aquas, but much more saturated, also don't look metallic. But there has to be more to it, as Bron suggests.

 

[chrono]

I am not sure what you mean by metallic. Can you please post an example?

 

[Daisys and Diamonds]

My guess is that it’s the presence of fine crystalline inclusions.
Faceting achieves sparkle by angled light return to the eye whereas crystalline inclusions will still reflect and refract light but in a haphazard way, dependent on their presence and position within the gemstone.
My wee Alexandrite throws tiny rainbow sparkles, it’s when light, at the right angle, creates a prism on the crystal inclusions.

you are so knowledgable Bron

 

[fredflintstone]

The combination of Spinel's hardness (takes a better polish), dispersion and a dark grayish color (or a light gray color) is the reason it can look metallic, but its luster is not metallic. It is sub-metallic to vitreous. Of course it depends on whose article you are reading, some articles say it can be subadamantine on polished surfaces.

So, there you go.

 

[CaseyLouLou]

The combination of Spinel's hardness (takes a better polish), dispersion and a dark grayish color (or a light gray color) is the reason it can look metallic, but its luster is not metallic. It is sub-metallic to vitreous. Of course it depends on whose article you are reading, some articles say it can be subadamantine on polished surfaces.

So, there you go.

That was clear as mud…just kidding. The gray undertone makes sense and I do think the high polish can looks reflective lending it a metallic appearance. They tend to be lighter in color and maybe some of them have more grey than it seems. I will post some photos as examples. Thanks!

 

[CaseyLouLou]

I am not sure what you mean by metallic. Can you please post an example?

Here are some examples.

The first ones aren’t mine but show as metallic to me in the photos and I have one that looks like this. Sometimes sapphires are described as looking metallic too. I have a silvery blue sapphire so that at least makes sense.



This next one is mine and in real life it sometimes in certain lighting has a very metallic look. It appears to be very highly polished so maybe that’s why. It sparkles like crazy too. It’s not a light pink but pretty saturated - medium. This is the one I’m puzzled by.

 

[fredflintstone]

That was clear as mud…just kidding. The gray undertone makes sense and I do think the high polish can looks reflective lending it a metallic appearance. They tend to be lighter in color and maybe some of them have more grey than it seems. I will post some photos as examples. Thanks!

Metal has a high polish, silver, platinum, gold, copper, even steel. Without that polish they would not look like metal as we know it. It would be a dull piece of ore. Same principal with Spinel.

 

[CaseyLouLou]

Metal has a high polish, silver, platinum, gold, copper, even steel. Without that polish they would not look like metal as we know it. It would be a dull piece of ore. Same principal with Spinel.

Makes sense. So I should appreciate the reflective property. Now I’m wondering what would happen to some of mine that have an amazing color but are kind of ‘soft’ looking on the surface if I were to have them polished more….

Like this one on the upper right. It’s sleepier but I don’t know if it’s the polish or the inclusions. I still like it and it does shine, just not to the same extent as others.

 

[fredflintstone]

Generally, it is only the very grayish stones that will look metallic. Not bright saturated colors.

Though not just with Spinels, but other hard stones the polish can be so fine they look wet in all colors.

 

[chrono]

From the examples you have shown, you are associating the steely gray with metallic. I agree with the other posters that a good polish helps with that too. You will never see this metallic with highly saturated spinel.

 

[T L]

The gem I own with the most metallic luster is my grey green diamond. Polish, which often is nice on hard gems, and the prevalence of grey, both help achieve this type of luster. I personally think luster, in of itself, is the most overlooked attribute of a gem. It really is a beautiful thing if polish is done well.

 

[theredspinel]

Here are some examples.

The first ones aren’t mine but show as metallic to me in the photos and I have one that looks like this. Sometimes sapphires are described as looking metallic too. I have a silvery blue sapphire so that at least makes sense.



This next one is mine and in real life it sometimes in certain lighting has a very metallic look. It appears to be very highly polished so maybe that’s why. It sparkles like crazy too. It’s not a light pink but pretty saturated - medium. This is the one I’m puzzled by.

Think what you’re calling metallic is just the tone of grey in the colour of the stone, all your examples of “metallic” have like a film of grey over the top.

Surprising to read someone actually prefers this colour way over seeing a more ‘pure’ colour like in your second example post with the pink spinal. A higher polish won’t introduce more grey into it.

 

[CaseyLouLou]

Think what you’re calling metallic is just the tone of grey in the colour of the stone, all your examples of “metallic” have like a film of grey over the top.

Surprising to read someone actually prefers this colour way over seeing a more ‘pure’ colour like in your second example post with the pink spinal. A higher polish won’t introduce more grey into it.

I think you might be right about the grey. I don’t actually prefer it but I’m trying to figure out why I sometimes see a metallic luster when I’m just not seeing the grey in my precision cut, high polish stone. My eye is not trained in gemstones!

My understanding is that metallic luster in gems is actually a higher ‘level’ of luster than others so I was guessing that to my eye an excellent polish is equivalent to metallic but maybe it’s just that there is a grey undertone in those metallic ones that I can’t always see.

I love my pure pink ones but I was just curious what would happen if they were polished a bit more.

 

[CaseyLouLou]

The gem I own with the most metallic luster is my grey green diamond. Polish, which often is nice on hard gems, and the prevalence of grey, both help achieve this type of luster. I personally think luster, in of itself, is the most overlooked attribute of a gem. It really is a beautiful thing if polish is done well.

Very interesting. It’s starting to make sense.

 

[CaseyLouLou]

From the examples you have shown, you are associating the steely gray with metallic. I agree with the other posters that a good polish helps with that too. You will never see this metallic with highly saturated spinel.

I think you are right. This helps me to know what I’m seeing and why.

 

[theredspinel]

I think you might be right about the grey. I don’t actually prefer it but I’m trying to figure out why I sometimes see a metallic luster when I’m just not seeing the grey in my precision cut, high polish stone. My eye is not trained in gemstones!

My understanding is that metallic luster in gems is actually a higher ‘level’ of luster than others so I was guessing that to my eye an excellent polish is equivalent to metallic but maybe it’s just that there is a grey undertone in those metallic ones that I can’t always see.

I love my pure pink ones but I was just curious what would happen if they were polished a bit more.

See if we use the correct terminology, vitreous luster, is typically how gemstone “luster” is described.

And that’s very much dependant upon the chemical makeup of the mineral, those minerals with relatively low refractive indexes (as well as being transparent/translucent) will have more of that glassy look to them over those with higher refractive indexes.

 

[theredspinel]

As a side note, the term metallic luster trips me a bit because usually that’s a term strongly associated with the Pearl world! It’s a mirror like shine pearls can have, attaching a picture from the net to show example.

 

[fredflintstone]

I think you might be right about the grey. I don’t actually prefer it but I’m trying to figure out why I sometimes see a metallic luster when I’m just not seeing the grey in my precision cut, high polish stone. My eye is not trained in gemstones!

My understanding is that metallic luster in gems is actually a higher ‘level’ of luster than others so I was guessing that to my eye an excellent polish is equivalent to metallic but maybe it’s just that there is a grey undertone in those metallic ones that I can’t always see.

I love my pure pink ones but I was just curious what would happen if they were polished a bit more.

Polish = Luster.

 

[theredspinel]

I think this link will answer most of your questions

What is Gemstone Luster? - International Gem Society

Gemstone luster describes how a gem's surface looks when it reflects light. Learn the common terms used for this property and what affects it.

www.gemsociety.org

 

[fredflintstone]

Luster (or lustre) is a surface gloss (caused by reflection), which depends to a large degree on:

• the refractive index.
• the ability to take polish of a gemstone.

The better the polish of the gemstone, the better the luster.

Since hardness defines what polish a stone can take, you can state:

• The amount and quality of the reflected light (which is called luster) is the highest for gemstones with a high refractive index, which also take an extremely good polish. In practice, these are usually the stones with the greatest hardness.

However, there is no direct relationship between hardness and luster. For instance: Sphalerite, while having an RI of 2.39, only has a hardness of 3.5 on Moh's scale, yet has a sub-adamantine (not quite diamond-like) luster.

The following terms are used to describe the luster of various gemstones:

1. Adamantine: the type of luster described by Diamond.
2. Sub-adamantine: gemstones having a high RI (but lower than Diamond), like Zircon and Sphene.
3. Vitreous: gemstones whose refractive indices fall within the range of middle values such as Emerald, Ruby, Spinel and most other transparent gemstones. Vitreous luster is sometimes described as "glass-like".
4. Resinous: gem materials that are soft and have low refractive indices, like Amber and Opal.
5. Waxy: an almost matt surface like that exhibited by Turquoise and Jadeite.
6. Silky: certain fibrous materials such as Gypsum and Malachite.
7. Pearly: the luster seen in Pearl, Moonstone and Talc.
8. Metallic: the very high luster shown by metals such as gold and silver, and by minerals such as polished Hematite.

According to Robert Webster in "Gems: Their sources, descriptions and identification", Dr. Hanneman proposed to give a numeric value to the types of luster. This never was put into practice, probably because of the more romantic connotations of the verbal descriptions.

Luster - The Gemology Project

gemologyproject.com

 

[peacechick]

This is my purple spinel, which I feel has a metallic lustre. But it’s dark and has no grey in it. So for me, it’s not based on colour. I think I might be referring to a feeling that the reflection is satiny?

I’m probably not using the term “metallic” correctly. But I do feel spinels reflect light differently from diamonds/zircon/sphere which are more scintillating and sapphire/tourmaline/garnets which to me have “softer” sparkle.

 

[fredflintstone]

This is my purple spinel, which I feel has a metallic lustre. But it’s dark and has no grey in it. So for me, it’s not based on colour. I think I might be referring to a feeling that the reflection is satiny?

I’m probably not using the term “metallic” correctly. But I do feel spinels reflect light differently from diamonds/zircon/sphere which are more scintillating and sapphire/tourmaline/garnets which to me have “softer” sparkle.

Spinel sits in the middle when it comes to dispersion (fire) between Diamond, Zircon, Sphene and Sapphire, Tourmaline, some garnets have more dispersion such as Demantoid and Mali Garnet.

Dispersion is not really an element of metallic luster, though it can be faux. Look at Platinum, Gold, Silver, it is shiny, but it does not sparkle.

 

[CaseyLouLou]

This is my purple spinel, which I feel has a metallic lustre. But it’s dark and has no grey in it. So for me, it’s not based on colour. I think I might be referring to a feeling that the reflection is satiny?

I’m probably not using the term “metallic” correctly. But I do feel spinels reflect light differently from diamonds/zircon/sphere which are more scintillating and sapphire/tourmaline/garnets which to me have “softer” sparkle.

This is interesting. I think I have decided that what I’m describing as metallic is more of a shiny, reflective property that some stones have and others don’t even if there’s no real grey. Maybe it’s not really metallic but it does remind me of a ‘foil’ look.

 

[T L]

This is interesting. I think I have decided that what I’m describing as metallic is more of a shiny, reflective property that some stones have and others don’t even if there’s no real grey. Maybe it’s not really metallic but it does remind me of a ‘foil’ look.

Here is a rough grey green diamond with a metallic luster. It’s almost iridescent.

 

[theredspinel]

This is my purple spinel, which I feel has a metallic lustre. But it’s dark and has no grey in it. So for me, it’s not based on colour. I think I might be referring to a feeling that the reflection is satiny?

I’m probably not using the term “metallic” correctly. But I do feel spinels reflect light differently from diamonds/zircon/sphere which are more scintillating and sapphire/tourmaline/garnets which to me have “softer” sparkle.

I would argue there was grey in this purple spinal!

 

[kgizo]

I once tried on a sapphire that had a “metallic luster”. It was beautiful. Regretted passing on it and reached out to a couple of vendors to try to find something similar and they had no idea what I was talking about. Is it stones with fine polish as some have suggested. Do certain colors in cool tones look like they have a metallic sheen? I wish I knew what caused this magic. These are screenshots from a video I took of the sapphire.

 

[CaseyLouLou]

Here is a rough grey green diamond with a metallic luster. It’s almost iridescent.

Yes that is kind of what one of my spinels shows. It is almost iridescent. A gleaming, reflective surface.

 

[fredflintstone]

At least in the picture, I see the Spinel as violet, not purple. The two can be confused.

Yes, it has a small amount of gray in it, at least in the picture. Think of top color Amethyst or Tanzanite, there you have no gray.

I would say, once again due to the picture, the Spinel is medium dark stone, not completely dark.

Very pretty Spinel all and all and fairly rare color, saturation.