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We recently learned in school that diamonds sparkle as the are very optically dense, meaning that it takes longer for light to pass through them, thus meaning that the light totally internally reflects when you look at a diamond, whereas in glass it would pass through at that angle.

Why have we not been able to create a solid material that is so optically dense? Is it because such materials are rare, expensive or hard to produce, or because no-one has looked into it?

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    $\begingroup$ Yes, it is. As far as I'm concerned, you can make Gold out of Lead; but it's not worth it. $\endgroup$ – M.A.R. ಠ_ಠ Dec 28 '14 at 18:42
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    $\begingroup$ As far as searching for "alternatives" or improvements for other things (e.g. bronze vs. steel vs. composites), improvement of one property is almost always is a tradeoff for something else. If you're making a structure of some sort, usually you just care about a couple properties (Young's modulus, tensile strength) and the others are irrelevant. Sometimes though, you care about lots of properties (color, castibility, patina, strength...) it's increasingly difficult. $\endgroup$ – Nick T Dec 29 '14 at 22:24
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    $\begingroup$ And for crystalline materials, you're talking about fairly fundamental atomic properties that are nearly impossible to rationally control or manipulate. $\endgroup$ – Nick T Dec 29 '14 at 22:27
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There are four properties that make a diamond look like a diamond. I will compare them with the two best diamond simulants at the moment:

  1. cubic zirconia $\ce{ZrO2}$ - a special ("cubic") form of zirconium oxide. Don't confuse it with zircon, a different mineral with the forumula $\ce{ZrSiO4}$.
  2. moissanite $\ce{SiC}$ - the name for the naturally occurring form of silicon carbide, also used for the synthetic gemstones varieties of it.

So what are the differences and why none of them is an all-round replacement for a diamond?

  1. Refractive index: determines the brilliance. This is what you were talking about when you said "optical density". This basically means the amount of light the diamond reflects to the observer. If you change the refractive index, you also have to slightly change the cut of the diamond to achieve optimal brilliance, and then your "diamond" will not look exactly like a diamond. The refractive index of diamond is 2.42. Moissanite is higher: 2.65 and cubic zirconia is lower: 2.17.
  2. Dispersion: determines the fire. This is what you wanted to say when you mentioned that diamonds are sparkling. Fire is the property of diamonds to break the light into different wavelengths thus creating the characteristic play of colors that you get in diamonds. Here's the interesting thing: The dispersion of diamond is 0.044 and interestingly the dispersion of both cubic zirconia and moissanite is higher: 0.060 and 0.104, respectively. So theoretically, if you consider only fire (the play of colors), then they both look better than diamonds.
  3. Birefringence: (the lack of) determines the clarity. When you look through a diamond at all angles, you can clearly see what's behind. This is also true for cubic zirconia. Both minerals are isotropic. On the other hand, moissanite is birefrigent - it will double the image you see through the mineral, causing it to look blurred and lose the visual clarity.
  4. Hardness: while not directly related to optical properties, a mineral with higher hardness will retain it's sharp cut. If the cut is damaged, the gemstone will not look as good and lose its value. Diamond has a hardness of 10, moissanite 9.25 and cubic zirconia 8.25. Note that this scale is not linear: 10 is much harder than 9 compared to 9 and 8, for example.

As you can see, none of the alternatives fully satisfies the entire set of properties that make a diamond. Moissanite may be slightly better, but it's also more expensive. And remember - one of the most appealing things about diamonds is their rarity and natural occurrence. No synthetic simulant can replace that.

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    $\begingroup$ huffingtonpost.com/rohin-dhar/… diamonds are bullshit (to some) $\endgroup$ – Jonny Dec 29 '14 at 7:12
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    $\begingroup$ @Jonny and it's not for others. The question was about diamond simulants, not whether diamonds are rare and whether they should be valuable. That said, I was never too fond of diamonds as gems: 1. They're (mostly) colorless and that's boring. 2. They're chemically too simple for my variety-in-elements-loving-brain. I'd rather have some tourmaline: Much more beautiful and also chemically interesting. $\endgroup$ – Gimelist Dec 29 '14 at 14:15
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There are plenty of synthetic materials that would fall into the "cheap diamond equivalent" category. If we're only talking about the optical properties, cubic zirconia ($\ce{ZrO2}$) is probably the most familiar as it is often used in jewelry. It's index of refraction isn't quite as high and it's not as hard, but it's close enough that a fair amount of it is used to make jewelry. A more recent development is moissanite (either synthetic or natural $\ce{SiC}$), which is almost as hard as diamond and actually has a higher refractive index. Other synthetic materials with even higher indices like barium titanate and gallium phosphide exist, though generally their optical properties are used for things like lasers.

Of course, we have gotten quite good at making synthetic diamond itself. They are only used for a tiny fraction of diamond jewelry, though. It wouldn't surprise me to find a concerted effort to market natural diamonds as superior, given how lucrative the gem diamond market is, which might be preventing wider use for gems. Synthetic diamonds are widespread in industrial applications, however, where smaller diamonds are used to make tools and optical properties are largely irrelevant.

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    $\begingroup$ Use of cubic zirconia and synthetic diamond in jewellery is (I think) an economics/psychology question rather than a chemistry one. Most casual observers won't notice the difference by eye, but one of the purposes of jewellery is to be expensive. So people who want expensive jewellery won't buy a cheaper version no matter how similar it looks, since it won't hold the same value. The natural diamond is superior because it's difficult/expensive to obtain, not because of its optical properties! $\endgroup$ – Steve Jessop Dec 28 '14 at 20:03
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    $\begingroup$ You're probably right. I imagine if diamonds were cheap enough to stick them on everything like rhinestones, they would probably lose a lot of their appeal. However, from what I can tell, gem-quality synthetic diamonds are actually more expensive than mined diamonds and are mostly used for coloured gems as coloured natural diamonds are very rare. $\endgroup$ – Michael DM Dryden Dec 28 '14 at 20:42

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