Mostly it consists of little cups and gaps that hold detritus that isn't broken down as efficiently. In an aragonite sand bed, bacteria coat each grain and the grains fit together more snugly to bring detritus and the bacteria together: happy, well-fed bacteria = nitrogen gas, which escapes upward to the atmosphere and no detritus. A CC bed has to be cleaned periodically and requires a fairly pricey piece of equipment to do it: you stir it up and run a special diatom filter to yank stuff out of your water really fast (because sandbed contents do not make your other critters happy, no more than you like to see the sewer back up into your dining room). This takes an hour or two.
Unfortunately, this process also stripmines your water of living microorganisms that your corals and inverts eat. So it's not an optimum procedure, even if it does make the water look nice.
