Toxic heavy metals. It’s highly unlikely they are naturally occurring.
All vanadium compounds should be considered toxic. Tetravalent VOSO4 has been reported to be over 5 times more toxic than trivalent V2O3.[74] The Occupational Safety and Health Administration (OSHA) has set an exposure limit of 0.05 mg/m3 for vanadium pentoxide dust and 0.1 mg/m3 for vanadium pentoxide fumes in workplace air for an 8-hour workday, 40-hour work week.[75] The National Institute for Occupational Safety and Health (NIOSH) has recommended that 35 mg/m3 of vanadium be considered immediately dangerous to life and health. This is the exposure level of a chemical that is likely to cause permanent health problems or death.[75]
Vanadium compounds are poorly absorbed through the gastrointestinal system. Inhalation exposures to vanadium and vanadium compounds result primarily in adverse effects on the respiratory system.[76][77][78] Quantitative data are, however, insufficient to derive a subchronic or chronic inhalation reference dose. Other effects have been reported after oral or inhalation exposures on blood parameters,[79][80] on liver,[81] on neurological development in rats,[82] and other organs.[83]
http://en.wikipedia.org/wiki/Vanadium