I think the largest disclosed un-certified gem right now is a 155 carat diamond. Which, sure, it’s a big gem. But if it was possible to make unreasonably large gems, we’d be seeing one-piece emerald kitchen counter slabs on the market right now and people with enough means would be making gemstone windows.
So, why is the largest synthetic gemstone still ‘only’ palm-sized? Is it a matter of scaling the size of equipment? The cost of materials? Some inherent fragility in the gems once they reach a certain size?
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The largest synthetic gemstones are actually **HUGE**.
We can grow some single(!) crystals such as quartz and sapphires that are way beyond anything natural, in the hundreds of kilograms, and that at an extreme purity. Or with a precisely controlled impurity to give them a certain color: pure sapphire is transparent like glass, add 1% chromium to make it a red ruby (yes, ruby is just a variant of sapphire), or add 0.01% iron to make it blue, and many other colors.
Alexandrite and emerald can also be grown quite well, but it is a bit more complex and thus more expensive. And unlike sapphire, they have a less wide use in industrial and consumer applications, adding further to the cost and lack of interest to optimize their size. Meanwhile, large so-called boules of sapphire are used for very sturdy “glass” (not really a glass), and ruby is used for lasers.
Diamonds however are quite finicky, they need quite extreme conditions to grow. Even more if one wants them to be extremely pure so that they are perfectly transparent. It is also not clear what a very large diamond is ultimately useful for beyond what the much cheaper sapphire can do; the little bit of extra hardness doesn’t do that much. There are however some applications as thermal conductors, as diamonds are the best one known.
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