What is the difference between this model and the MGUC500?

I called the company to ask. They said that the MGUC500 is the newer model, and adds multiple preset times (the CD-2800 automatically runs for 3 minutes with no options). Despite the supposed retail prices listed here, the company gave the same prices as Amazon: about $40 for the newer MGUC500 and $30 for the older CD-2800 without the presets.

Is it safe to clean semiprecius gemstones?

Absolutely yes. I am a jewelry designer, who works with precious and semi-precious gems, including rubies, sapphire, emeralds, quartz, citrines...etc.. and the ultrasonic works gently and thoroughly, using sound waves. There's nothing abrasive or rough. I clean my expensive tri-focal, treated designer eyewear and lenses with it, too. It's sensational. I went through a few machines, until i tried this one by Magnosonic, and it is the best one for the price that I've used. I would recommend it. You can easily clean all your jewelry with it, and it does silver beautifully. I use a little cleaner in it, too. The combination is super.

Can you clean make up brushes in it?

Yes. I have cleaned makeup brushes, hairbrushes, combs. Anything that can’t be ruined.by water or heat.

I have a lot of jewelry that's tarnished, will it remove sterling silver tarnish?

Ultrasonic cleaners don't remove oxide or sulfide tarnish; however, you can do this with kitchen chemicals. Search YouTube for "How to Remove Tarnish from Silver Jewelry" or similar.