Actually Janet's nipple is pierced. The sun nipple shield that she was wearing is open in the center and a delicate bar passes through the nipple's horizontal hole to secure the shield in place.
I have seen a picture of her wearing a doorknocker type of ring what is called a "captured bead ring".
I saw the show in a bar in Italy and it was really late, when I saw this stunt I thought for sure she was wearing the doorknocker ring...but upon closer inspection...I am just glad that I don't have to explain this to a den of boy scouts and girl scouts.

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Sex is a tricky thing best left to professionals.