The you diatomite you describe sounds like the stuff i used to get, before orica closed the mine.

I get the coal ash from centenary landscapes, the last lot i got was $120m3. I normaly put it through a 3/4" sieve & I use the big chunks with limed pine bark for my orchids. i store it in 500L bunnings plastic water tanks to keep the weeds etc out. The mixed media i keep in wheelybins.

I think the thing is to find what is available localy & works.
I would love to try scoria if only for the look but its $375m3 here & i would rather use the money to build my new cactus house since my bunnings greenhouse didnt survive the last thunderstorm too well.