"... their weight is gold" is correct. Singular "they" and "their" has been used for centuries. Here is a BBC article about singular "they." https://public.oed.com/blog/a-brief-history-of-singular-they/ Also, here is an italki discussion about "he," "he/she," "he or she," and "they." https://www.italki.com/question/487356

You should use "their" to generally refer to someone as either "his" or "her". Just remember that you should never address people as objects. Of course there are exceptions where you use "this" to address people, like introducing yourself on the phone, "This is Alyosha." or introducing someone to others, "This is my brother, Alex." but never "it" or "its".

Better to use "his or her." "Their" implies employeeS (more than one). The pronoun should match in number and gender to the subject noun. Hope this helps.

Their, you don't refer to people as it