I'm looking for the name of an old sci-fi book. In it, a spaceship’s crew goes to a planet to mine a large deposit of diamond. However, they find an alien entity encased in the diamond. They also go to the center of the galaxy in the story.
Space Angel by John Maddox Roberts
An independant trader starship learns of a wilderness planet with an outcropping of valuable diamond slab. They discover a metal sphere inside the diamond. This turns out to be a billion year old weakly godlike entity who needs their help. A rival entity is Occupying the galactic core, and the first entity needs transportation.
Along the way they enlist the help of two "Vivers", who are warlike mutated humans. They also get help from an alien crab like being who is a poet.