These links will give you some insight into different lighting set-ups that will give different results in coin photography. The third link, in particular, has tips for shooting coins in slabs and for getting good results on toning.
http://www.tabletopstudio.com/docum...tography.htmhttp://www.sigma-2.com/camerajim/cjgcoins.htmhttp://browncopper.com/photography.htmlI've personally found that photographing toned silver works well with just the on-board flash and some very white cardboard/paper to reflect the light from the opposite side of the coin...just to reduce shadows. You absolutely want to avoid having "too much light" (i.e., overexposing the photo or having exposure correct "on average" but with "hot spots" where there's too much light) because that tends to "drown out" the details, including subtle toning. The other thing to be sure of is the white balance setting on your digital camera. If it is too warm or too cool, you'll create unnatural yellow to pink to blue casts to your coin's appearance. You may find using some software to correct for white balance and contrast can help get you an image that accurately portrays to coin's true color, too. The key to post-processing, however, is to show your coin accurately as opposed to enhancing it to appear a better grade than it actually is.
Here's an example taken with the onboard flash and some bounce cards:


Since you are shooting through a slab, you want to be certain that the surface is very clean and unscratched. Tilting the slab slightly to reduce glare or reflections could help, but you may then need to use a smaller apperture (higher f/stop) to increase depth of focus or run the risk of one edge or the other of the coin being out of focus.
Good luck, and show us some of your results. We might be able to look at what you're achieving and give some specific advice!