wireless optical mice,either laser led or older or newer tracking technologies, still have issues with very glossy surfaces.
Here's something creative you could try.
Use a digital camera with no flash, take a picture of the granite surface.
Import into your iMac using iPhoto and print it out on sone heavy weight matte paper.
You may have to edit the image to get it as close to the real granite top as possible.
So, print this out on some regular ink jet paper first.
Use a 8-1-2 "x 11"sheet to print out the image. Cut the sheet to a size you think would be fine as a working area for the mouse (say a 6" x 6" or 8" x 8" area)
Locate the area you took a picture of and lay the sheet in place until you find the exact area.
Mark the edges with some masking tape.
Then use double stick tape on the back of the sheet and tape in down to the exact place you took Yhe original image.
You now have a flat, non glossy image of the granite desk area to run your Magic Mouse on.