I bet the scoria you use is just as good or better than pumice as a drainage aid, it is so rough and porous. Most of the ornamental palms grown in Hawaii and imported to the states use the crushed lava rock in their mixes. (I bet they don't have to go far to get their lava rock
)
While on the subject of crushed granite, I recently re-roofed my patio with torch down roofing and I have 3/4 of an 80lb sack of biege colored roof granuals left over, the bag states it is crushed granite and upon close inspection looks as if it is highly pure. It is very clean as nothing will wash out of it when rinsed. Can this material be used as a seed cover to help prevent damping off when using organic germination media or is it too course? I'll guess the particle size is +/- 1/32"-1/16" on average and is very uniform. I don't want it to go to waste if is good for something, it was reasonably inexpensive at $18 for an 80lb sack.
Keith