What is the ruling on trading in diamonds through the International Diamond Bank, where an individual deposits a sum to purchase a raw diamond, and the bank processes it and sells it to stores after the investors' approval, then the amount and profits are returned to the investors after the bank takes a 5% commission from the profit for its mediation and account management?

There is no objection to purchasing diamonds through the bank, cutting them, polishing them, and selling them, with the bank receiving a percentage of the profit in return for its work. However, two matters must be noted: First, it is impermissible to sell what has been purchased before taking possession of it. The possession by the bank, acting as a Mudarib or agent, stands in place of possession by the principal. Second, the relationship between you and the bank could be a Mudarabah (definition) (profit-sharing partnership), where the capital is from you and the work is from the bank in exchange for a known percentage of the profit. Mudarabah is permissible provided that the worker (the bank) does not guarantee the capital. It could also be an agency for a fee, which the Hanbalis have permitted. It is imperative to ensure that the transaction is real and not merely a formality, so that it does not become usury (riba).