The Jeweler is an anthropomorphic duck (although, judging from the shape of his beak, he may also be part goose).
The Jeweler is, as his name plainly states, the friendly owner of a jewelry store. He wears a blue or red robe similar to Scrooge McDuck's, a pink or blue bowtie, pince-nez glasses, and white spats. His physical appearance is similar to that of Gladstone Gander. He appears to be a friend of Gyro Gearloose, because he had Gyro make him a burglar scarer known as "Inspector Clumper".
Behind the scenes
The Jeweler calls Scrooge McDuck "uncle", implying they may be relatives (though close friends of the Duck family, such as Mickey, have been known to call him as such as well). If that is the case, his resemblance to Gladstone Gander, coupled with his apparent wealth, suggest he might be none other than Gladstone's mysterious brother and the father of Shamrock Gander, though that is pure speculation.