You need to file a police report with your local law enforcement agency.

Oh, I will go to the police, but I think I will go into the store first to see if the manager has had other complaints about him, and maybe find out the guy's name so that I can give that to the authorities. They might not give me his name, though. Still, I think I will make up some signs and stand at the road telling people that AT&T installers steal from your home. This violation has really gotten under my skin, and I will not let this go soon.

If the police believe that a crime was committed, they will be able to get his name during their investigation.

Have the facts (date/time/place) of the alleged theft when you file the report.

The store has no relationship to the technician's shop.  You'd have about as much luck walking into a Verizon store.

Good news. Here’s the story:

I had recently put a couple of fat, stubby quartz points and a petrified wood branch out on the livingroom bookshelf next to the TV. The missing larger smoky quartz had been in the living room bookshelf, the other bookshelf, on the mantel, all over the living room, and I didn’t worry too much about it. I buy these things because I like to look at them and I don’t keep track of them. My mother was dusting the shelf and noticed a crystal that she wasn't used to seeing there, so it stood out in her mind. Then the installer came and left. I looked around for my smoky quartz point, and it was nowhere to be found. I took all my crystals and things away, and I freaked out a bit. I asked my mother if she had moved my smokey quartz point, she said no. I showed her the stubby quartz points that I had taken away and asked her if these were the ones she was dusting, and she said no. She remembered dusting a much taller crystal point. Well, that clinched it. That's my missing crystal. So, I got his contact information, and wrote him a email telling him that as sure as my mother was that she was dusting it before he arrived, I was sure that he had taken it. I hadn’t gone to the authorities yet because I wanted to give him a chance to return it. I figured that if I went to AT&T and caused a fuss, he would have to deny it, and I would never see it again. So I put the pressure on him, and hoped he would do the right thing. I had a witness whom I trusted. I know I owned it. It was no longer there. He was the only one who had been there since. Two plus two. Simple deductive reasoning. Just to make sure, I turned out my closet, looked under my bed, in cupboards, even under my bathroom sink just to be sure. Nowhere. Then I went into my mom’s painting studio and on her bookshelf, behind a decorative candle holder, acting as a bookend, was my smokey quartz point. It turns out that she not only forgot that she moved it who knows when, but what she was dusting was what I had shown her. Sigh. I feel terrible now. So, I wrote the guy another email appologizing, and told him how sorry I was for accusing him. I really feel bad about it, but I was working with reliable testimony of someone I completely trust. Still feel like a chump, though.

It takes a big man to admit when he's made a mistake....It takes an even bigger man to apologize.  You did good.