Why is he always looking at my necklace?

Hi !
So this guy and I are in the same class at Uni and every time we talk to each other (once a week) he looks at my necklace for a few seconds. I know people say men use this tactic to look at the chest, but I mostly wear turtlenecks. So I really don't understand.
Why is he always looking at my necklace?
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