One of the very few things that have never changed about Zoro's character design is his earrings. On his left ear, he wears three identical gold earrings. While on the surface they look like just another accessory that one would wear, however, they certainly do carry some significance.
The three earrings are an indication of the Three Sword Style that he has mastered over the years. It is highly likely that Zoro will always wear these earrings even when he is much older, as implied by Eiichiro Oda's character designs of him at the age of 40 and 60.