Several people have asked this and I thought I would put together something a bit more comprehensive than "use macro mode". If anyone has anything else to add or information below to edit, please feel free to weigh in.
CAVEAT: This how-to is geared towards people using point and shoot cameras, if you are using an SLR or a DSLR, you can overcome many of the shortcomings of a point and shoot by manually adjusting the settings.
Diamonds are very challenging to take photos of - especially appealing photos. This is even more of an issue when you use a point and shoot camera with autofocus.
When you use a point and shoot autofocus camera in macro or "flower mode" (close focus), it has a very narrow depth of view (plane of focus). In macro mode, the camera has a very shallow depth of view. A bit of photography 101 here - The depth of view is the plane perpendicular to the camera that is in focus on the resulting image. This can range from very wide (landscape mode) to very narrow (using macro or high zoom). The depth of view is dictated on the ability of the camera to gather light. Wider in bright outdoor areas, and very shallow when you''re up close to something or at night time. So, depending on how the camera interprets the incoming light, the shallow plane of the diamond that is the most appealing, may not be the one that ends up in focus. Think of this as a very thin slice of your diamond that the camera can focus on. This is because what you see (the light reflected off the pavilion of the diamond is then projected and refracted (scattered) off the surface as it leaves the stone (giving you fire), so ideally, the camera would focus on the external table of the diamond (what your eye sees which is a projected image of the pavilion, as opposed to the pavilion itself), but most of the time it actually focuses on the pavilion since the table is essentially a window into the diamond when looking at it from straight on.
I didn't find the right solution from the internet.