It never had any value to begin with. It was a product where the marketing got ahead of the development. Some smooth talking salesman found himself in possession of the stuff, silver-tongued his way through a huge sale by promising all sorts of valuable properties to the metal, and it caught on. The material started changing hands, shot up in value as a untested commodity because it's so shiny, it has to be great, right?
Large organizations (banks, corporations, government factions) caught wind of the hype and started "Go west, young man" campaigns to get cheap labor in on the ground floor. Promises of wealth and a solid career in the MGM industry brought lots of men and women out to seek their fortune. Soon warehouses all over the map were full of the stuff.
Then the metallurgists got a few samples. 159
People were bringing in nuggets, ingots, anything they could get. All were asking for the miracle devices they were promised to be crafted. Weapons, armor, machine parts, the works.
It turns out it is only useful as an alloy to other metals, usually metals that are so scarce, large-scale practical uses are too far out of scope for the amount of materials any one person or organization can realistically possess. However, the metallic mix is close to 80% MGM and 20% Unobtainium.
Or the process of making the alloy is so time consuming or dangerous to not be worth the effort.
Either way, many small trinkets exist, horseshoes, ashtrays. Whispers exist of a family that has an entire cutlery set made from MGM and Unobtainium (Or just plain MGM if that's what you go for, if so, ignore the next paragraph).
Because of the scarcity of the Unobtainium, MGM values have plummeted to nil. Sure, some people still run mining ventures because the occasional investor can be duped into thinking that when Unobtainium becomes Scarce-but-still-possible-to-obtainium the future of MGM will blow through the roof.
As for now, the abundant surplus of MGM sits uselessly in warehouses, frequently in ghost towns. The value of the material is so low it's not worth paying a few laborers a day's work to move the stock.
The door the party encounters is clearly made of either masterfully crafted MGM or the 80/20 mix. 446