I took this pic with my Canon EOS 550D, using a tripod and artificial lighting. I don't understand why the image is blurred when viewed at 100%. There was no digital zoom. What should I do to get a perfectly sharp picture? How do I benchmark my images against the actual capability of my camera?
Welcome to the wonderful world of macro product photography.
First, you need to understand that when you photograph things at high magnification, you won't get a whole lot of depth of field. In this instance, very little is in focus. Look at the leftmost two pendant rings at the bottom of the earring on the right: just that little bit is sharp, and most of the rest of the image is out-of-focus to one degree or another. You would need to use a smaller aperture (bigger f-number) to get more in focus, and take several images focused at different distances and merge them using a technique called focus stacking to get all of it in focus. There's no way around the physics; you won't be able to get everything in focus at the same time unless you reduce the magnification at the sensor by using something like a compact camera with a much smaller sensor.
There is another problem here as well, and that's that the earrings are very much overexposed. You are going to get blown-out highlights unless you let everything but the sparkles (specular highlights) get very, very dark. Again, that's unavoidable in a single image. But when overexposure gets to this level, it causes problems both within the lens (flare and purple fringing) and at the sensor (because colour information for each pixel depends on neighbouring pixels, and badly overexposed pixels carry no useful colour information). To get both sharp earrings and a readable background with the light you're using now, you will probably need to take at least two images and merge them to an HDR image. Or you could use broader, flatter light to illuminate the background objects (primarily, it will also fill shadows on the earrings and provide some highlights), which will also have the effect of reducing the relative intensity of the light that's providing the sparkle. (You can get the fill light from your main light simply by using cardboard or tin foil reflectors if you don't have separate lighting available.) Don't be afraid to spend some time taking pictures and playing with the light. There are shortcuts (like "light tents"), but they almost always result in uninteresting pictures.