My first trial involved the only rocks I had, a few stones from my mineral collection. I had plenty of those and I decided a little heat and frankincense wont't kill them.
This is a magical hazard as well as a health hazard. Sulfur drives most spirits away, even demons, and calls saturnine influences. Also, sulfur smoke is hazardous to breathe.
I cannot caution our practitioners that ventilation is vital to the ritual space. If the space is too small, not only the smoke becomes unbearable, but the coals can smolder away some nasty carbon monoxide.