If something is jewelled, it has jewels — real or fake precious stones — set into it, sewn onto it, or covering it. A jewelled crown, a jewelled pin, a jewelled box. Writers also stretch it a little: a night sky can be “jewelled with stars,” meaning it looks as if stones were stuck there. This is the British spelling; American English writes jeweled. It is an ordinary descriptive word, a bit dressy, not slang.