A necklace for a guy is a little bit different than one for a woman.
For one thing, men are never going to wear anything that could be considered “costume jewelry” — the big, chunky stuff with rows of jewels that you’ll see accompanying evening gowns. That’s strictly a woman’s fashion.
There’s also much less of an idea among designers that the jewelry should be the centerpiece of an outfit. Men’s jewelry is meant to complement, not to overwhelm.
That means necklaces that are a little more rugged and rough around the edges. Good jewelry for men should look natural — like you’ve been wearing it for years, after it was given to you by your grandfather, who wore it for years before you.
You’ll see some brighter, shinier stuff, of course, and even the massive “bling” of urban caricature. And there’ll be some very light and fragile designs here and there too, but at either extreme you’re talking about exceptions to the rule. They catch people’s attention precisely because they’re so outlandish.
Most men who wear necklaces well won’t be noticeable for their jewelry. You’ll just see them as “stylish,” and then notice the accents over time.