I do prepress
work at a print company for a living, so I have a
good pointer on sharpening images that many of you may not know. Instead of using unsharp mask, you should convert the image to LAB color. Instead of R,G, & B channels, there will be L, A, & B. The L channel is the lightness/darkness. The A and B involve the colors. Now, select the A and B channels and apply a little
bit of gausian blur to "smooth out" the color from any JPEG compression artifacts. Then, sharpen the L channel. You can sharpen the L channel quite a
bit without making the image looks like its been over-sharpened. It really produces superior results with a
bit of experimentation. Then, you can convert it back to RGB mode without any color shifting. LAB mode is the most powerful color space for any kind of color-correction or sharpening.
Once you get a feel for the blur and sharpen settings that
work well for the photos you take, you can even record the whole shebang as an action, so that you can process entire batches at once. I use actions all the time at
work. An example you might use is if you want to make all of your coin images the exact same size. Open the Actions pallette, have a coin image handy, and click the little paper-with-turned-corner on the bottom of the pallette. That's for "create new action". Name it whatever you want, then it will automatically be in record mode (if not, click the
red circle). At this point, you can change the image size to whatever your
standard is (or any other things you'd like to automate). when you're finished, click the gray square to stop recording. Then, when you have an image open, all you have to do is go to the Actions pallette, select the action you created, and press the green triangle. It will do all your stuff for you automatically. Even better, go to File>Automate>Batch and you can select a whole folder full of images and have it run the action on all of them while you sit back and sip your coffee. It's a real time-saver!
As for taking an odd color
cast off of silver without affecting the color of the rest of the coin, I'd use an adjustment layer (curves) in combination with a layer mask. You can use the layer mask to control which
part of the image is affected by the adjustment layer...and to what degree. The best thing about adjustment layers is that if you save the image as a TIFF or PSD, the adjustments you make are completely reversable or tweakable at any time in the future.
In the layers pallette, click on the little half-black, half-white
circle at the bottom. That's for creating the adjustment layer. It'll make a menu
pop up with the choices of what kind of adjustmest layer you want to make (curves, levels, etc.) once you create it, do your settings and press OK. You don't have to make it perfect at first, because you can go back and tweak it any time you like. You'll notice that there is a white box in the layers pallette in the adjustment layer. That is the layer mask. If you click on it, you can use things like the paintbrush or the gradient tool to determine what parts of the image are affected by the adjustment layer. Anything painted black will be unaffected, anything white will be fully affected. Shades of gray make things partially affected. This is very useful...and completely reversable/editable at any time. You can save as a TIFF or PSD and go back and tweak your adjustments weeks later if you feel like it. Or, you can create multiple adjustment layers and turn them on or off to compare them and see which one you prefer.
If you want to avoid that, and simply want to adjust one
part of the image by selecting it and doing a quick-and-dirty adjustment, the best way to do it so that the line between the adjusted and non-adjusted
part is not noticeable is to make the selection (with any of the selection
tools, such as the lasso...or painting it in quick-mask mode), then going into quick-mask mode (if you are not already there) and using gaussian blur to "feather" the selection
area. This gives you an excellent amount of control with the feathering. A lot of people don't realize that the "painted" areas in quick-mask mode can have filters
applied to them. You can do some very nice effects this way, such as creating vignettes with edges that look like rippled
glass. Try it out...it's fun.