Do some studies of those things. Notice how they mostly appear as noise, you can't really tell apart the specific grains or pebbles. If you squint your eyes it will appear as a uniform surface for the most part, with a single, or several values, creating the illusion that there's gravel there, getting the brain to fill in the blanks.
If you're working digitally you can easily use a brush of some sort and the right colours and values to make it feel right. Working with traditional media is a bit harder and time consuming but the general idea is the same, you try and get it to feel right.
If you need brushes you can find a bunch of them here: http://digitalbrushes.tumblr.com/
Most of them are PS but there are some for Gimp too (and I think you can get the PS ones to work with some fiddling).
And here's mine, the two-point one while fun was done kinda half-assedly, will probably try and do something more interesting tommorrow.