cam, if i remember my science classes correctly (they were a long time ago) volcanis sand, rocks, and ash have high amounts of ferrous metals in them. lets not all forget that lava is nothing more than melted rock and ore and all the other cool stuff found in dirt. alot of impure minerals and substances will burn off leaving the stuff less likely to dissapate; ie irons. as lava rock has been used for years with out ill effect, i think it unlikely that your volcanic sand will be all that bad. just my thoughts though.
while, magnetite could and probably is in the sand, the pics of the mag show what looks to be actual shards or splinters of metal. they just dont look like natural occurances. my instinct is that either you have had some sort of failure in your tank like suggested, or the manufacturer had a failure at his plant and you happened to get some contaminated sand.
