Do you have a cite for the existence of golf-ball-sized diamonds? Public information on synthetic diamonds is very hard to come by. However, the news stories that I’ve seen (like this) indicate that the largest gem-quality diamonds are in the 3 carat range. I have occasionally heard stories of golf-ball-sized synthetic diamonds, but none with any corroboration at all.
Around $600 is the cheapest to be found, we looked hard too.
None except for my own eyes. Our weekly physics colloquium one Friday was given by a geophysicist who studies minerals under extreme pressure conditions. To generate the pressures, she uses a diamond-headed press, and to be able to observe the samples she’s studying, she needs the press to be transparent. She brought along one of the diamonds she uses, and it was huge. Golf-ball-sized is an estimate, of course, but it was definitely much bigger than 3 carats.
I wouldn’t put too much weight in the “news story” you linked. It’s pretty obviously an advertisement that the company convinced the NYT to run for free. And when you cut through the advertising BS, it looks like what they’re claiming is that it’s the largest grown diamond indistinguishable from a natural diamond in the world. It might also be the largest sold for purposes of jewelry, but that’s just because if anyone saw jewelry with a stone as big as the one I saw, everyone would assume that it was fake.
Sadly, quite a lot of “news” falls in this category. I linked to it only because there’s almost no information on the subject. The best article I’ve read on synthetic diamonds was one on Wired, but it’s from 2003 and obviously out of date. The largest gem-quality diamonds they mention were also 3 carats, and yellow.
Still–I’ve read about diamond anvils before (used for research into metallic hydrogen as well), but with no indications of a diamond that large. They are usually smaller than a carat. There’s not a tremendous reason to have anything larger, because you only need them for very small samples in the first place. The pointy end is limited by the compressive strength of the diamond, and the blunt end only needs to be large enough to match the strength of the rest of the press (hardened steel, say).
Obviously I believe that you saw what you thought was a synthetic diamond that large, and maybe the geophysicist thought that as well. But are you certain that it was diamond and not, say, synthetic sapphire? Sapphire is almost as strong as diamond and can be made in 100+ kg large chunks (large enough to slice off thin sheets for use as LCD screen protectors). It might even make sense to have most of the anvil made from sapphire and some small critical portion made from diamond.
Sorry, I’m not trying to be jerkish here, but “size of golf balls” is a hell of a claim, and while by no means impossible, I haven’t seen any evidence that it’s been done.
You’re right that information is surprisingly sparse, but here’s a cite from 2005 of a group making 10-carat diamonds, and anticipating 300 carats.
Nice thread. So OP owes alleged blowhard a mental apology if this anecdote accounted for his putative blowhardosity.
300 carats would definitely qualify, but actual confirmation of this still seems to be nonexistent. The Wired article also mentions the possibility:
4-inch wide diamond wafers would be fantastic, but I can’t see that it’s happened. It should be big news in the semiconductor industry, which I pay some attention to, but there’s virtually nothing. To the extent that it is used, it’s mostly in very thin CVD films, even though one of the big things about diamond is its huge heat conductivity, and semiconductors would benefit greatly if the entire die was diamond. $5/carat would be amazing; a 10x10x0.5 mm wafer would be <$5 and easily pay for itself with its performance advantages.
Superman’s least called for superpower. 
Y’know, it occurs to me that if I could make 300 carat diamonds, I’d probably keep a low media profile about it, too. Diamonds that big would be worthless as jewelry, because they’d look fake. And the publicly-known existence of such gems would drive down the price of diamonds of three-ish carats, which can be sold as jewelry. There might be a few industrial uses for such stones, but the profit you’d make selling to that niche would be much less than what you’d make selling for jewelry, and besides, you could still sell a few for industrial purposes without putting stories in newspapers or online about it.
One can be factual and still be a blowhard. The guy just never shut up. Interesting information, all.
I can’t agree. The semiconductor industry is something like $350B a year. Diamonds are like $13B.
Large, perfect, diamonds would be a huge boon for semiconductors, and probably be worth a double-digit percentage of the total market size. There are always numerous news stories when semiconductor manufacturing transitions to a new wafer size (the latest is from 300 mm to 450 mm), and likewise for any new semiconductor material.
I think you’re right that the secrecy is self-enforced, but at the least I’d say it’s because their output isn’t enough for major industrial use. Maybe the big diamonds exist for highly specialized uses but not in the realm of millions of units. Still, I’d think that there would be stories of labs using them.