I've only seen pumice stone used in (sheltered) gas fireplaces, idk I'm not really that much of an expert, I was told what I put in the comment when I was a child and brought some rocks for a fire pit my uncle was building and he said 'No! Those will explode, look for some like these or these' pointing to some granite and a big quartz (or quartz-like, I ain't no geologist) chunk. The couple dozen (at least) times I've done it without an incident and a quick web search I've just done confirm that my folk knowledge is right.

I think you might be smoking crack.

That was absolutely uncalled for, Steve.