Visit the Diamonds International Shop in the port of Roatan and also located in Roatan Cruise Ship Terminal.
Image courtesy of Diamonds International on Trip Advisor
These guys are all over the Caribbean and as such are a major chain so you can be sure of the autenticity of their claims in terms of the metal.
Find out more on the official website - http://www.diamondsinternational.com/location/store/honduras
Facebook page for the DI in Roatan - https://www.facebook.com/DI.Roatan/