It was Steve Truevol, the cyborg butler. Chip knew it was the butler after he made the mistake of saying it was the diamond that was stolen and not the ruby. Only the thief knew the diamond was stolen; everyone else thought it was the ruby.
Faced with the facts, Steve confessed. He revealed that he was the victim of Kevin Kon’s first con, which is how he ended up needing cybernetic replacements. In order to get revenge, Steve conned the con man by becoming his loyal butler. Steve planned to use the diamond to finance his travel blog.
You don't need to be a private investigator to know that The Dogboy Chronicles are available at Amazon.