I did an experiment with my compots. I used hydroton (a different brand of prime agri) alone, hydroton mixed with arkolite (a locally available diatomaceous rock-not round), and hydroton mixed with arkolite and lava rock (larger chunks-for air movement).
The compots were all of the same plants. (L. purpurata WCL x Mc. brysiana 'Heckler's')
They were all in the same area of the greenhouse under the same lighting condition and watered on the same schedule.
The compots of hydroton and hydroton/arkolite mix have insane growth and are almost ready to put into seedling pots.
The compots with hydroton, arkolite and lava rock slowly went downhill. I lost 5 plants in that compot and none in the others. They have been in the compots since November 2010. I finally removed the seedlings from the lava rock compot last night and repotted them into hydroton mixed with arkolite.
Now this was not a scientific experiment and of course you can expect to lose some seedlings from a flask anyway, but it pretty much confirmed that the local volcanic rock available at Lowe's/Home Depot is not really good material for my compots.
Now I will say that I use this same rock in the very bottom of some of my bigger s/h pots as weight and I don't see any adverse effects on my established orchids. That being said, when I repot those I will be removing the lava rock and I will just use an outer heavy pot to steady the plastic s/h pot if needed.
If the rock you have works for you, definitely go for it. I would like to see photos later on for comparison.