GABORONE, Botswana — Botswana President Duma Boko has called for a strategic alliance with Angola to protect the value of natural diamonds and strengthen the bargaining power of the two major African diamond producers in the global market. President Boko said Botswana and Angola should deepen cooperation in the diamond sector as part of efforts to restore the industry and ensure that countries producing the precious stones derive greater value from their mineral wealth. He ar