What object left the cross-shaped shadow on Becquerel's famous radioactivity plate?

The object that left the cross-shaped shadow on Becquerel’s famous radioactivity plate was a Maltese cross.

Becquerel placed a metal Maltese cross between uranium salts and a photographic plate wrapped in black paper. The uranium radiation passed through the paper and exposed the plate, but the metal cross absorbed or weakened enough radiation to leave its recognizable silhouette.

The experiment was initially connected to Becquerel’s interest in phosphorescent materials and the newly reported X-rays. He expected sunlight-activated uranium compounds to produce an image, but the cross appeared even when the setup had been kept in darkness. That unexpected result helped establish that uranium emitted radiation spontaneously.

The cross was not an image drawn by light and it was not a later symbol added to the plate. It was a shadow-like radiographic impression produced by differential absorption of radiation. Some diagrams call it a copper Maltese cross or a medal, but “Maltese cross” is the canonical answer to the question.