Can I use landscaping lava rock as a concrete slab base?
By itself I would not rely on it, as this is lava rock which isn't the strongest - it's prone to breakage..  However, if you cover it with stone dust to help fill in the voids which will help prevent shifting (which encourages breaking) and then solidly compact it with a power compactor, then follow up by covering it with a layer of sand on top, compacted again, then I think you'll have a good solid base for your slab.

What really matters, though, is how deep it goes.  You need the base to go down below the normal frost line for your area.  If you don't dig down deeply enough, then come winter, the freezing ground under the slab will heave and destroy your concrete, which will be a huge waste of money.

So if you need to dig down more, which you'll find out quickly (if you're in N CA go 16-24", if you're in the southern part, 1' will be fine) then dig down, and fill with this first, then gravel, then stone dust and finally sand - compacting each layer as you go.