There are a fair few “rock & lava” type bases just done with broken IKEA cork place mats.
@Rusty_shackleford has a really distinctive ‘comic book’ paint style (which various people seem to copy 
) but the basing works very well.
If you want something more “real” then buy the cork matting, but break it up a lot, stick it more randomly then maybe give it a coat of textured paint (wilkos do a couple for example), just to disguise things a bit.
I’ve not done an army in this style yet, but would probably do a halfway house vibe