DAVE THE DIVER

Mjolnir is found inside a rock that randomly spawns in the Glacier Zone. It will look like an aquamarine stone. By breaking the stone, you can pull Mjolnir out of it.
Once you open the glacial zone, you can notice a cluster of ice with Mjolnir inside. To break it you need to use a fully upgraded dive knife. A fully upgraded Diving Knife allows you to mine anything and also increases its power.
After breaking the ice cluster, interact with Mjolnir. You will have to pull it to the side and, while pulling, go through a QTE
Then simply kill any nearby fish.

On foreign websites, players write that you have a better chance of finding a hammer during stormy weather. I searched over 20 times in normal weather and found nothing. Then during the storm it was in two places. just restart until the storm hits and search the area.

Video guide (en).

When I put the necklace on, the buff to minerals immediately appeared. I almost didn’t even pass by, since it was larger than an ordinary stone and more transparent, it seemed like it was just something in the background. And it came out in the most ordinary place, next to a stone with crabs.