Based on the wildlife and the general geography of Valheim, the area in which it takes place appears to be similar to the Norse nations, specifically ones along the North Sea. The North Sea and Danish coasts are perfect for oysters to grow, as the oysters are built for that type of water. Thus, oysters (not mussels, as the North Sea and Danish coasts are saltwater) should be able to spawn in the water and slowly grow, and mature oysters should have a chance of having a pearl inside. The chance of finding a pearl should be rather low, as it would require a mature oyster in real life up to a year to fully develop a pearl. For gameplay, it would likely be far faster growing and a much higher rate than the real life percentage, as in real life less than .0001% of fully mature oysters in the wild contain pearls, almost all pearls coming from commercial oyster farming in which a nucleus is inserted into the oyster (or mussel) to allow it to slowly be coated by aragonite and conchiolin, the substance secreted which creates the pearl. These nucleuses are circular so the pearl will be so as well. In the wild, this is instead achieved due to the presence of a parasite, which irritates the oyster or mussel, resulting in it being coated as a defence mechanism. The result is almost never spherical, though it would likely be made spherical ingame for the sake of simplicity. Oysters produce millions of eggs, thus it would make sense for them to naturally spawn near the coast, on piers, or on large rocks underwater. Naturally, opening one would likely require a blade or a blunt weapon, collecting the oyster only giving you the oyster itself. It would also function as a good food source. For gameplay’s sake, it would likely be between .5-10%, and finding mature oysters in general would likely be rare. As per time, I believe a few in game days would work as the amount of time it takes for them to mature.

All in all, I believe that the addition of oysters would greatly benefit Valheim, providing a very good food source (oysters in real life contain all 9 amino acids and have 9 grams of protein per 100 grams of serving) and a source of a potential rare material, of course being pearls. It would also increase the biodiversity of the seas, adding an interesting and useful creature with very different mechanics than all other existing marine life in the game, and making exploring the water more rewarding.