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Comment by thaumasiotes | Hacker News Reader

> Moissanites have already become as near-worthless as synthetic rubies.

I've noticed that synthetic sapphires are (or were?) much more expensive than synthetic rubies. Do you know why?

I'm going to take a guess and say it's because of ruby lasers. They make massive synthetic ruby rods for lasers, but it needs to be one solid piece with very low impurities. A small defect will cause the rod to be entirely unusable for a laser, but there can still be large portions usable for other purposes with less stringent requirements. An example is ruby tipped 3D printer nozzles, used for abrasive filaments, which can be had for around $50.

Other mass-market uses for small rubies are for bearings in precision gear trains, and wear-resistant tips for precision measuring devices.

Why would those need to be red?

I'm also curious about this. Especially since natural rubies and natural sapphires are the same type of gemstone, just in different colors. It sounds like the synthetic equivalents might not be similar at all!

The synthetic equivalents have to be similar, because they're the same thing as the natural stones, and the natural stones are similar.

A ruby is corundum with chromium coloring it; a sapphire is corundum with (most typically) iron coloring it.

Iron isn't rare, so either it affects the process used to make rubies, or there's no real reason for the price gap.

Iron is not enough for sapphires, it must be combined with titanium.

I do not know how this is done, but while the doping with chromium for rubies is simple, chromium will just substitute aluminum in the crystal lattice and it will provide a color determined by its concentration, for sapphires there may be necessary a more complex thermal treatment, so that the iron and titanium will form the right type of coupled defects in the crystal, in order to have the desired color.

"No real" reason wouldn't shock me. Not a gemstone expert so I can't say, but if natural sapphires are more expensive/in higher demand than natural rubies (despite both being corundum) then it makes sense for that demand to map onto their synthetic counterparts. But with the synthetic stones, it's more obvious how arbitrary that demand/price difference is.

My impression was that natural rubies are generally more expensive than natural sapphires.

This is just speculation, but assuming you're talking about gem-grade rubies and sapphires, perhaps there's more industrial uses for similar rubies and the gem industry sees the benefit.

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