It can't hurt. If they do have to provide insurance under state licensing:

-You contacted the company in good faith.
-You paid for the services in good faith, and have the receipt.
-The man they needed to investigate stole from you.
-Therefore, you should be compensated, preferably by finding out where the jewelry was pawned.

BTW, you'll probably be asked for a list of anyone and everyone who was in your house from the day you last saw the jewelry until the day you noticed it was gone. It's a good idea to have that list ready, and if it includes any strangers, interrogate them, also!

If I were you I'd contact your Homeowners insurance company and put them to work. Then in turn should go after the plumbing company with their own claims and possibly legal people if necessary. I'd make them do all the work instead of doing it yourself.