Gold and diamonds are more valuable materials - the most valuable, as a matter of fact. For Valjean, the candlesticks are symbolic of two things. First, the freedom from parole and persecution that was bought by the Bishop's generous "gift". Second, the deliverance of his soul to God that was brought about when the Bishop granted that gift. For a man who was accustomed to the selfish and hard edge of human nature only, the forgiveness of the Bishop and the words of love - calling Valjean his "brother" - were a weight lifted. Valjean was lead from that point to become a selfless and moral man, atoning for past sins. Therefore, the candlesticks, by saving his soul, are the most valuable thing in his life.