Andre Norton 
Zero Stone 

A gem trader and a feline mutant unite to find the truth behind a mysterious stone rumored to be the key to all the power in the universe.  Apprentice gem trader Murdoc Jern lives on Angkor, a thriving hub of interstellar trade. One day, a furtive visitor brings him a ring discovered on an alien corpse. It’s made of age-pitted metal surrounding a dull and lifeless stone. Yet it exerts a powerful hold on Murdoc. And when his father is killed, Murdoc begins a quest, taking with him his only legacy, the mysterious ring, which he knows is the key to his dad’s death.   Accompanied by Eet, a feline mutant with paranormal powers, Murdoc hurtles through space, where he’s pursued by a religious order called the Green Robes. His journey takes him to distant planets and once-great armies marooned on vanished worlds. In a desperate battle to learn the ultimate meaning of the Zero Stone, Murdoc must bargain for his freedom—and his life—and confront the fate that befalls seekers of knowledge.