Your iPhone is a great camera for taking photos of jewelry and other possessions for your records including for insurance purposes.    It is also a great camera for taking photos of objects that you wish to sell.  Here are some tips:

Shooting Area.  Lighting is an important element.  You want to use natural light so that you don’t have to use flash.  A table in front of a window on a bright but cloudy day is perfect.  Add lamps for additional lighting if needed.  Check your photos early to insure you are getting detail.  

Background.  Use white tagboard or paper as a background.  If you are selling the object, you can also consider using black to show off your item.  Some experts suggest that silver looks better on black and gold looks best on white.  

Get Close.  You want to show detail  in your photos.  Remember, Zoom with your feet.  Get close.  Zooming in will make items appear flat.  

Multiple angles.  For insurance or appraisal purposes take photos from multiple angles so that no detail is missed.  Experts recommend photos from three different angles, for example, from the top and two photos from the side.   Remember, it doesn’t really cost anything to take lots of photos to get the detail.  

Backup your Photos.  Keep your photos in more than one place,  Use cloud storage (iCloud, Google or others off site storage) to backup your photos.