Synthetic bulk and natural pyrite from the hydrothermal mine in Schönbrunn (Saxony, Germany) are confirmed to be stoichiometric FeS₂ compounds and stable (for thermoelectric applications) up to ∼600 K by combined thermal, chemical, spectroscopic and X-ray diffraction analyses. Natural pyrite with a small amount (<0.6 wt%) of well-defined transition metal carbonates revealed characteristics of a nondegenerate semiconductor and is suitable as a model system for the investigation of thermoelectric performance. In the temperature range 50–600 K both natural and synthetic high quality bulk FeS₂ samples show electrical resistivity and Seebeck coefficients varying within 220–5 × 10⁻³ Ω m and 4 – (−450) μV K⁻¹, respectively. The large thermal conductivity (∼40 W m⁻¹ K⁻¹ at 300 K) is exclusively due to phononic contribution, showing a well pronounced maximum centered at ∼75 K for natural pyrite (grain size ≤5 mm). It becomes almost completely suppressed in the sintered bulk samples due to the increase of point defect concentration and additional scattering on the grain boundaries (grain size ≤100 μm). The thermoelectric performance of pure pyrite with ZT ∼ 10⁻⁶ at 600 K is indeed by a factor of ∼1000 worse than those reported earlier for some minerals and synthetic samples.