The Zero Stone Series by Andre Norton

A mysterious stone, born of worlds long extinct, is the key to unimaginable powers in this latest novel from the winner of the Science Fiction Grand Master Award. Murdoc Jern and his mutant feline companion, Eet, are hunted throughout space when Murdoc comes into possession of a magnificent gem.

The Zero Stone…that most priceless of alien artifacts, had been the death of Murdoc Jern’s father, and might soon become the ruin of Murdoc himself. It was the Zero Stone which had led him into a partnership with the catlike alien, Eet. And it had been the Zero Stone that had seen them caught as pawns between the Thieves’ Guild and the interstellar force of law, the Patrol... moreThe Zero Stone…that most priceless of alien artifacts, had been the death of Murdoc Jern’s father, and might soon become the ruin of Murdoc himself. It was the Zero Stone which had led him into a partnership with the catlike alien, Eet. And it had been the Zero Stone that had seen them caught as pawns between the Thieves’ Guild and the interstellar force of law, the Patrol. And while Murdoc and Eet had somehow survived against all the odds – had even wrested enough credits from the Patrol to legally buy a spaceship – their troubles were only beginning.For now they must seek their fortune along the uncharted fringes of inhabited space in a search for the world from which the Zero Stone had originally come. But others, too, sought this place which hid treasures beyond imagining. And these unknown rivals, as well as the Guild and the Patrol, were only too ready to sacrifice one insignificant human and his alien ally if they got in the way… less

"Murdoc Jern's father, an interstellar gem trader, was murdered by outlaw competitors and left behind an odd ring, large enough to fit over the finger of a space suit. With his small companion Eet, a strange feline mutant with phenomenal mental powers, Murdoc soon discovered that the stone in the ring was actually a Zero Stone - an alien device left behind by an ancient vanished race - and it was the key to powers beyond human imagination... more"Murdoc Jern's father, an interstellar gem trader, was murdered by outlaw competitors and left behind an odd ring, large enough to fit over the finger of a space suit. With his small companion Eet, a strange feline mutant with phenomenal mental powers, Murdoc soon discovered that the stone in the ring was actually a Zero Stone - an alien device left behind by an ancient vanished race - and it was the key to powers beyond human imagination." Murdoc and Eet had to solve the secret of the Zero Stone, and very quickly, because very greedy and dangerous people wanted that ring, and wouldn't hesitate at a second murder to obtain It. And Murdoc couldn't count on the Patrol for help, because they, too, were pursuing him for reasons of their own. He and Eet were lone players In a game whose outcome could mean life or death, both for them and for billions of others throughout the galaxy. less