Hi Rie and welcome to the forum. So you thought that look in your profile pic would get you a faster answer, eh? Well you were right!
Unfortunately that’s not an artefact, or at least wasn’t made as such. It’s a natural “ironstone” concretion, most likely composed primarily of goethite. Despite the high iron content, it’s unlikely to be magnetic unless it has any magnetite as a secondary mineral. These concretions are frequently hollow and depending on their original shape and how they break, they may resemble a cup or bowl. Here’s a piece that’s a less fortunate shape.
The colouration on the inside is frequently taken as an indication that such items have been used as native American paint pots, but it’s just the result of oxidation of the iron minerals. It is entirely possible that such items may have been used as containers by the finder, but that couldn’t be claimed without some context.