diamond facets are really a bunch of mirrors. They need to be lit well from the front when photographing... all the light comes from the front, it is how they are designed! and you need shards of black and other colors/shades/shapes to look l "realistic". Funy thing, in real life they often don't, but not in photos! The set will look like a hodgepodge of cut mat board placed here and there--and light crowded in... it is long and tedious work. Either your client is hoping you can pull it off, or the photographer is... then there is the DOF issue. IN order to get everything in focus, he either has to use a macro lens ( which is what I expect he/she has used) and using a focus sled, provide many images that can be joined together ( an automatic procedure) called focus stacking, or I have found using a wide angle that can focus very close like my 25mm Zeiss Distagon and a D800e helps avoid this problem.
Fixing completely blurry images and picking out facet detail from badly lit gem stones is not accomplished at the retouching stage. Improving and enhancing those things is- but you need something to work with! I expect that the photographer was had at a deal, or shot by someone who thinks its easy and now they expect you to pull it off.
Walk away- nobody will be happy with your results.