Honolulu Jewelry CompanySterling Silver Turtle Honu Ocean Sea Life Necklace Pendant with 18" Box Chain

How wide is the hole on the pendant in case i want to switch chains?

Is it really sterling? I just bought one in Cayman and after wearing it for a week, the So called sterling silver rubbed off to copper.

what size is the turtle

Is the underside of the turtle unfinished or does it show a detailed shell on the bottom as well?