Posting work in my Etsy shop has made me a better photographer. There, the photo is everything. A good photo will make your jewelry look even better. A bad photo will make even good work look bad. On Etsy, there are 5 photos to show off your work, so the first one shouldn't be about showing the whole piece, it should be about enticing your audience. It should be an interesting and amazing photo on its own. The space they provide is a horizontal rectangle, so I always shoot horizontally to get the most out of the space.
Rings and pendants are easy to take because they fill up that little rectangle so well. If a pendant is quite long, I'll take the photo so that is lies diagonally to fill up the frame.