If they were an employee of the company, the company may be liable under a negligent hiring, negligent supervision, or negligent retention theories among others. The employer has some duties as to their employees. Also, you may have a claim for general negligence against the employer based on respondeat superior but that appears unlikely in your case.
Probably so, under one theory or another. Of course, you have to prove it was stolen. I would give the company an opportunity to make it right. If they fail to amends, then it may not hurt to hire an attorney.
The above statements are provided as general information and not intended as legal advice. Each matter has its own set of unique circumstances that cannot be adequately addressed without consultation. You are strongly advised to hire an attorney licensed to practice law in your state to represent you.
Generally, the company would be responsible for your losses if the person who stole the jewelry was an employee. It would come under the theory of negligent hiring and supervision. If the company knew of a prior problem with the same worker, it would be evidence of negligence. Make sure you file a police report first.