Crushed Lava
 

Could anyone (particularly those in the Hosuton, TX area) have any ideas
where to find crushed lava for use in potting mixes. I've checked most of
the horticultural and stone/gravel/sand supply places around town and no one
has any idea. I can find large decorative chunks - do I just have to crush
it myself? YIKES!

Thanks!
Blake in Houston