If I remember correctly Harry's pentagram belonged to Margaret, or so he keeps saying.. It is one of the few physical things supposedly handed down to him from her. But then again Thomas also wears a pentagram identical to the one Harry wears, well not quite, his now has a road map gem of the Ways in it. So she must have had them made for her boys.. As far as we know, Thomas isn't starborn and Margaret wasn't starborn, yet they wear and wore a pentagram.
The people who wear a pentacle in the series to date are Harry, Margaret, Thomas, and Elaine. Harry wears it both because it was his mother's, and because he believes in what it represents, thanks to his grandfather: Magic confined within a circle of human will. Given what we know about Margaret, it seems likely that she wore it for reasons similar to Harry, because it was what she believed. Thomas pretty much wears it only because it is a gift from his mother. He even goes into how he doesn't see magic the way Harry does, in Back Up. And we have no insight into why Elaine wears it, but I think she was wearing it back at Justin's as well. We know so little about her past, or her current way of thinking, that it becomes hard to assign motives there.
