Sharpness is absolutely critical in product photography. You're expected to make consumers want to buy the product and muddy, soft images aren't going to get that job done. This is particularly true with items that have lots of small details to bring out, like jewelry. Lack of sharpness plagues many novice photographers in this genre and the reason why is often hard to understand.
For many inexperienced photographers, a lack of sharpness is often thought to be due to shallow depth of field, so they stop down more and more, while the problem persists and can actually become worse. It's a frustrating situation and it's usually due to one of the most misunderstood tendencies of light: diffraction. It's an issue that all photographers need to be aware of and it can be particularly frustrating in the product photography studio, where minute details are important.
Light travels in waves and when those waves are forced to travel through a small opening like a lens aperture, they scatter. This scattering effect causes interference with the adjacent waves and this results in a loss of focus when the waves reach the camera sensor. Generally speaking, the smaller the aperture, the greater the diffraction effect. This means that the minimum aperture isn't necessarily going to give you the maximum sharpness, regardless of the depth of field.
To illustrate this effect, I recommend you take the time to watch this great video from PHOTIGY. It's a little bit of a lengthy watch, but a great demonstration of the relationship between aperture size and diffraction.
Did you notice the huge studio and impressive setup used in that video? Here's some good news: you don't have to duplicate that to take great product photos. Plenty of pros create stunning product images with a tabletop setup that needn't cost a fortune.
One that we like to recommend is the MS20 Kit from MyStudio. It's a complete setup, with literally everything you need to create professional quality product photos, it's compact, portable and it comes with a price tag that almost any photographer can afford. Do yourself a favor and check it out: