Dear RL:
You wrote:

>Regarding the same issue, can I use a singular verb if I want to modify “jewellery sector”?

> The jewellery sector in Italy is one of the key manufacturing industries that HAS sustained constant growth in exports.

>Thank you again for your kind attention.

In this case the clause would be modifying the word “one” since that is the closest singular word to the clause. However, since “one” is standing in for “jewelry sector,” then the effect is the same.

>”Precious metal,” can it be plural?

>Thanks again.

If you are speaking of more than one variety–for example, silver and gold–then you would say “Precious metals.”