I am asking the material of the prop, not the hammer in the universe. I am asking the material for the hammer tip, handle and strap.
There's an interview of Russell Bobbitt, Hollywood prop master, about that in this Screenrant article. There are actually several props, depending on the nature of the scene it's in. Rubber, fiberglass, you name it.
So how many hammers are usually on set? You see them flip it around and obviously it seemed lighter and then you see the real one and it's still fairly dense. It's pretty heavy.
Bobbitt: So all the different materials I use for Mjolnir. I have a metal one which is about 60 pounds and then we do a fiberglass one. We do a hard rubber and a soft rubber. The soft rubber is for stunts, if he's throwing it or catching it or hitting somebody or it hits somebody, we use a very soft rubber. The softer the material, the harder it is for me to sell as metal because it's more porous and it's harder to paint. The metal one looks beautiful because it's metal. The next one down is fiberglass. I can put a really beautiful paint job on a fiberglass hammer. That's the one he'll carry around a lot, you know, if he has, if I see that he's going to have a 12-14 hour day, I'm giving them something lighter because it's, eve if you're the perfect specimen that Chris Hemsworth is, and I'm sure lots of people will agree with me when I say that ,it still gets heavy after 12 hours, no matter how strong you are. So I provide lightweight ones and rubber depending on the stunts and whatnot. And I will, just a little trivial fact the paint for that fiberglass hammer to make it look like the real metal hammer and make it look like Uru metal, is $1,000 a gallon. That's a crazy long process, that takes two to three days, and that doesn't count the labor. If there's anything expensive out there, I find it.
There's also one prop which is specifically for the "character X can't lift Mjolnir (because they're not worthy)" scenes. This one is intentionnally heavy, for realism purposes. It's not specified what it's made of, but presumably not rubber.
Bobbitt: But I also use old materials - not just 3D printing. Captain America’s shield shouldn't always feel light - so I use aluminium so the actor feels the weight of the piece.
With Thor’s hammer I get one that’s 80lbs so the actors have one they that can’t lift for some scenes. It adds authenticity and realism.
Back in 2012 (phase 1 wasn't even finished!), Marvel sold some props at auction. Below are the descriptions given in the catalog for the auction, made by Profiles in History. As they're a bit worn from production use, they were worth the modest sum of roughly $3,000 - $6,000 each.
STUNT MJÖLNIR WAR HAMMER FROM THOR
Stunt Mjölnir, Thor's distinctive large, square-headed war hammer, with foam rubber head realistically painted to resemble ancient forged metal, entwined Nordic Design and Runes, and stout handle wrapped in brown leather with lanyard. Head measures 8 ½ in. x 5 in., 17 in. long overall.
[Item 212, Mjölnir stuck in rock] Exhibits minor cracking and wear from production use. $4,000 - $6,000
[Item 213, Mjölnir wielded by Thor] Exhibits an indentation on one end, some cracking and wear from production use. $3,000 - $5,000