You can use someone else's idea for jewelry - ideas cannot be copyrighted - however you can not copy someone else's design (the way they have developed the idea).
An example: A jeweler uses emeralds and diamonds in his ring designs, creating images from the gems to make unique gem-pictures. I can use emeralds and diamonds, too - but my ring designs cannot copy or be substantially similar to the other jeweler's designs (the shapes of the settings, the arrangement of the gems, anything unique that sets the other jeweler's designs apart from the rest - those elements can be protected by copyright and should not be used). If my rings are believed by the other jeweler to be a copy of his unique design, or substantially similar to his design, I could be sued for infringement.