Again, adamantium is not an element, it's an extremely strong steel alloy. At least it is in the comics and the previous movies (the movies didn't specify steel per se, but they did show it was a specially mixed alloy). The MCU has certainly changed things from the comics before, but turning adamantium into some kind of alien element would be an odd change.
I mean, the only reason it's possible to make things out of adamantium is that it doesn't become nigh-indestructible until after its ingredients are mixed and solidified. If it started out as a pure, nigh-indestructible element, it would be nigh-impossible to melt, forge, and shape it. So it only makes sense as a human-made alloy rather than a naturally occurring mineral like vibranium. (Which is something that really bugs me about The Mandalorian. If beskar is so strong that it can even survive a lightsaber, how the hell is it so easy to melt down and reforge?)