Gravel or grit is something we all use at one time or another as modelers. It covers out bases, fills gaps in scenery, I even use it to weight the underside of bases sometimes (just infill the hollow area under the base for added base weight). Unfortunately the grit sold in model shops (usually in various grades for model railways) is expensive and comes in rather small bags. Alternatives like builders sand or beach sand can be used but have to be washed thoroughly and allowed to dry before you can pot them up.
My solution is to use Calcium Sand from my local pet store. It’s the type of sand that they put in the bottom of Reptile cages and is naturally a Gray colour (similar to primer) but also comes in various pre-died shades. As an alternative to other sources it is cheap, comes in a bag big enough to last you a very long time (possibly forever if you only us it for figure basing) and needs no preparation.