The paper describes the construction in some detail. It's an aluminum foam with ceramic spheres embedded in it. They then welded into a thin steel box to give it a uniform surface. If you watch the video, you can see that the grinder gets through the steel box very easily, but the deeper it goes into the aluminum, the faster it is worn away. The grinding wheel is destroyed in just a few seconds, which is pretty impressive. On the other hand I doubt that this will be useful for smaller objects like locks.

It says it right in the text of the article

"An angle grinder or drill bit will cut through the outer layer of a Proteus plate, but once it reaches the embedded ceramic spheres, the fun begins with vibrations that blunt the tool's sharp edges, and then fine particles of ceramic dust begin filling up gaps in the matrix-like structure of the metal."

Ah, ok. Looking at the composition on the side seems to indicate that it's two separate metals entirely. Which I guess is the case?

If you don't want to wait for someone to answer, the short article has answers for all the questions you put forth so far.

I did read the article, but I don't see anywhere it states that there will clearly be one material on the outside and then a block of proteus underneath. What they are grinding is clearly two different materials, and it stops when it gets to the second one. The x-rays and the example don't seem to have a metal outside welded on, which is my confusion - it looks like someone welded a plate onto the miracle material, when in that little image in the corner of the x-ray it looks entirely homogeneous.

Because we can't directly see the miracle material, the video feels disingenuous.

It's not clear to me either when exactly does the "cutting" start, but note that when he finally gives up the grinder wheel is an inch smaller.

Thank you - I was having a hard time understanding why the video was a successful demonstration. Re-watching with the diameter of the cutting wheel in mind was illuminating.

Looks like it was a 3mm thick plate.

Given that this stuff is half as dense as Aluminum, it's pretty impressive that the grinder behaves like it's cutting the most nasty steel (steel is ~3x the density of aluminum and much tougher).

If the plate had been aluminum, the grinder would have cut through like it was playdough.