The article "una" is crucial here. "La gioielleria" behaves the same way "jewellery" does. It refers to jewellery in general. "A piece of jewellery" would be "un gioiello". Hence "una gioielleria", with the indefinite article, must always refer to a jeweller's (shop).
Boy, I'm really having trouble with spelling. So many double consonants. Duo is kicking my butt, He won't give me any leeway whatsoever. I guess it's the best way to learn, but boy, I'm having a hard time.