Paul Phillis

Iron may not be a problem to plants but other more hazardous metals may be present and any elemental sulfur, seen as a yellow powder or a smell like rotten eggs, would be of serious concern.

Not all volcanic rock is stable when kept continuously moist as in bonsai pots, where any free metals can leach out of the porous material.

Test it first by growing other less important plants in it, not your prized bonsai.
Paul