Jewelry in English is a collective noun and is treated as if singular, so you would only say "is beautiful". You could say "pieces of jewelry are beautiful" though because then the "are" refers to the plural word "pieces".
Here is the danger of translating too literally. "תכשיטים" is plural in Hebrew, but the English equivalent "jewelry" is a collective noun treated as singular. Each language is it's own system, and has to be understood as such.