The gasket does deform. But that doesn't let the pressure out.

I'm not in this field, but I know several experts in it. I've helped out on experiments and have loaded DACs myself up to 20GPa or so.

I'll describe how it works in practice at that kind of pressure, and then what I understand of the different techniques needed for the extreme pressures used in this paper.

Loading the DAC is fiddly and done by hand. You start off looking through a microscope at the bottom side of the DAC. The lower diamond is pointing up at you. The tip of it is truncated at the point to give a flat end face around 100um across. With tweezers and a steady hand, you place your gasket - basically a rhenium washer with a maybe ~50um inner diameter - on top of that, with the outer edge of the gasket overhanging the edges of the diamond tip.

Then you need to drop two things into the hole in the middle. A little fragment of your sample material, and a tiny piece of ruby. The ruby is your pressure gauge! Ruby fluoresces, at a wavelength that varies with pressure in a known way. So when the cell is loaded, you can shine a laser through the ruby and look for the spectrographic line of the fluorescence to get the pressure.

Now you have your sample and your ruby sitting inside the gasket, but they're surrounded by air. That's not going to work. You need the space around them to be filled with a liquid, to act as a medium to distribute the pressure. So you take a little syringe and plop a drop of whatever your chosen liquid is into the hole. There are different things you might use depending on the experiment.

Now that you have everything in there ready, you can attach the top side of the DAC and bring the second diamond down onto the top of the gasket. The cell will need to have been carefully aligned beforehand to ensure that the diamond tips meet up flat in the middle.

With everything together, you can now start applying pressure. In simple DACs this is literally just a case of tightening screws around the outside by hand. For more precise/automated control, you can get cells with a built in gas membrane.

But your question was about the gasket - what role does it play? Well, firstly you need something there to hold your sample and your ruby and your liquid in place while you prepare and load the cell. If you just pushed the two diamonds together the liquid would just flow out, the diamonds would hit the sample or the ruby and crush them at a point, rather than compressing them under hydrostatic conditions.

But you need the gasket to give way when you start applying pressure, which is what it does. It gets thinner, and material is pushed out of the sides. When you eventually unload the cell, you can look at what's left of the gasket and see the imprints of the ends of the diamonds in it.

You still need the gasket to be made out of something very strong so that it doesn't pull apart completely. But it's playing a very different role to the diamonds. They need to be hard. The gasket needs to be tough.

The diamonds, by the way, usually sit in tungsten carbide seats, because that's hard enough to take the pressure transmitted through the wide end of the diamond, which is still pretty high. The carbide seats then widen out further to the point where the rest of the cell can be made from stainless steel.

But that's all at the measly 20GPa or so that I'm vaguely familiar with.

What I got from chatting to an expert about this paper was something like the following.

425GPa is crazy pressure. That's four million atmospheres. At this point you're actually beyond the compressive yield strength of diamond. If you tried to use flat-ended diamonds like I described, they'd shatter before you got half way to that pressure.

Instead, they use a focused ion beam to machine a recess in the face of each diamond, giving what's called a toroidal cell. You can actually see that in the compressed shape of the gasket in Figure 1a. The force is distributed in a ring around the outside, so that it stays within the range of what the diamonds can handle.

The sample in the middle is being compressed from above and below by the diamonds, and also from the sides by the gasket material being pushed inwards by the raised rings on the diamond faces coming together. So you're actually using it to help pressurise the sample even though you're way past the yield strengths of both materials.

It will have required some incredible capabilities to design, build, align and load that cell. Even aside from the question of metallic hydrogen, there's some serious showing off here just in terms of techniques.