I had an idea for a necklace in my story that did the same sort of thing as the signature center gem of the Sword of Omens from Thundercats (pictured above), where there's a big colored gem with a black pattern inside or behind the gemstone. But google has so far failed to produce any evidence that anything like this design can be done in real life with actual precious gems, as opposed to colored plastic or glass, so I wanna double-check to make sure this is something that can actually be done and that I'm not indirectly insinuating the gem is a fake.
It does magic things, right? You can do anything with magic. Like bond a bunch of real gems together I would assume.
Funnily enough, while there is magic in this setting, as well as magic jewelry, the gem in question is not intended to be magical in nature.
Well, I'm no gemologist, but they tend not to be very big. Even one the size of a pinky nail is considered to be "colossal." So I guess it's possible as with the Hope Diamond, which they say is walnut sized at 25mm. That's huge for a gem but not like huge-huge.
It's not the size I'm worried about, it's the question of if real gemstones are translucent enough that a symbol put behind one in such a fashion would be clearly visible.
Is your question about the actual setting if the gem in the sword or the creation of a dark or opaque design on the surface of the gem? My guess is this might be possible with say Jade, but even if it were, the reason you don't see it is it probably reduces it's worth. It might even be possible with diamonds using lasers or something that has a hardness comparable to diamond (which is rare) but again, why would anyone in real life do it?
My impression is that the symbol is intended to either be behind or inside the gemstone, not on top of it, though I suppose that would be another way of doing it.
I just googled "How translucent are rubies?" and the info I found says they're fairly transparent but not completely, and things are seen a little distorted and cloudy through them. I also did it as an image search ,and while I didn't see any rubies sitting on top of something so you can really test it, you can often see the back facets through it, and those look pretty distorted and blurry. The gemstones in your pic of the sword aren't faceted, they're dome-cut, which would introduce some more distortion. Basically it's a big lens, so anything seen through it would be very distorted, and as it moves the distortion would shift with the movement. Something like this: But if we're talking real this-world gemstones, they wouldn't be nearly as transparent as a glass of water. Of course in a more legendary land they might have gems as clear as tinted water—that's entirely up to you. I mean, the Conan stories were set on our Earth, just in a special Hyperborian age long in the past, and they had all kinds of crazy jewels the size of human skulls. There's no rule that says the laws of physics need to be identical in every story to the ones we're familiar with. That would take a lot of the fun out of a lot of stories.
If it's just the eye pattern, something like that actually occurs naturally. It's called chatoyancy, or literally the Cat's Eye effect, where the gem has a discoloration in the form of a band running through it. Main difference is, as far as I can tell, that they are brighter than the stone itself rather than darker. (I don't know for sure if this is always the case, though.) A similar effect is seen in star sapphires and star rubies, where additional bands form a star pattern: Otherwise, I think the most basic way of achieving what you want would be to cut the red stone in two (or just get two of them), grind them so that there's a space between them in the desired shape, and insert a black gem in that space when you mount them on the sword. Difficult, but probably not undoable with the right tools and expertise. If it necessarily has to be one single stone, then it gets trickier. I'm pretty sure you can change the color of gems using heat, but I'm not aware of any technique that could achieve something that precise. If we're talking about a fantasy setting, I suppose having it done via magic would be the obvious approach. Or you can have it be natural like the ones pictured above but say that the band is some exotic mineral that doesn't exist in our world. Anyway, at the end of the day, the only people likely to call you out on this would be actual geologists. And they can be easily dealt with.
I think you could easily build a cavity that would be 50 μm undersize in diameter then heat the opening—being careful not to let the blade heat as it would ruin the temper. You could then place the stone in the opening and let it cool to what is called an interference fit. I think most gem level stones would handle the temporary heat as the opening cools.
On the one hand: creative licence. The proportion of readers who are qualified minerologists is very small, and the proportion of those who see errors like this all the time in fantasy books and just shrug is very large. On the other: we write what we know. If we know about gemstones, we should write about them - our insight into any topic grounds the fantasy in the real world and helps readers tolerate our unfortunate compulsion: to describe imaginary countries at the expense of character. But if we don't know about gemstones, probably we should choose something else to flesh out our storyworld.
