JOHANNESBURG, Jan 10 (Reuters) - The Board of Debswana, the 50/50 diamond mining joint venture between De Beers Group and the government of Botswana, has approved investment of about $1 billion for a key development phase of the Jwaneng underground project, it said on Wednesday.

The investment will pave the way for the world's most valuable diamond mine to transition from open pit to underground operations, the statement said. (Reporting by Felix Njini and Nellie Peyton; Editing by Louise Heavens)