I may have referenced this before in past posting of my work and such, but there's really only a very few materials I use for basing my figures - one reason being is that that the materials I use are generally inexpensive, and another is that the stuff I use covers a lot of (ahem) ground in terms of potential flexibility in appearance for basing figures. Here we have the collected contents of several large ant mounds, mostly separated out - a material I like to call "ant dirt."
The above picture is the first step in the process of "making" custom basing material - or texture material, if you prefer that name. It might actually be the second step or so, but I don't think the prior processes (read: me scraping up ant mounds from the gaps in my driveway and picking out bits of vegetation) rate a picture. What you see on the paper plate in the image is a loose pile of collected dirt and gravel from an ant mound.
Using the mesh sifter in the above picture, I essentially folded the paper plate into a "funnel" of sorts and sifted the dirt and gravel through the mesh. I found the best results were to pour a thin stream of the dirt and gravel from the plate down through the angled sides of the sifter, which generally allowed the finer grit to pour right through and shifted the larger pieces towards the middle. Keeping the mesh over the larger container and lightly shaking, I finished separating out all the fine grit; I can later use it for a variety of effects - rust texture, moss, fine grit, loose rubble/sand, and 28mm scale dirt. What ended up left in the mesh filter were the larger (reasonably speaking) rocks and gravel, which I dumped into a separate container for later use as, well... fine scale rocks and gravel.