α-PbO2–type TiO2 (space group Pbcn, a thermodynamically predicted high-pressure polymorph of rutile) was synthesized by a number of investigators using shock and static compression experiments. Recently, in situ high-pressure and high-temperature studies employing the multi-anvil device and white-beam (using a synchrotron radiation source) energy-dispersive method indicated that the transformation pressure is lower for nanophase material (∼4 GPa and 900°C) than for the bulk (∼6 GPa and 850°C). In addition, the phase boundary of rutile/α-PbO2-type TiO2 changes from a negative to a positive slope with increasing temperature. This timely knowledge provides indicative pressure-temperature (P-T) constraints on the natural occurrence of α-PbO2-type TiO2, recently identified by analytical electron microscopy as an epitaxial nanometer-thick slab between twinned rutile bicrystals in almandine-rich garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. The stability field of “bulk” α-PbO2-structured TiO2 shows that the minimum stabilization pressure of transition is ∼6 GPa and could have been up to 2 GPa lower as a result of the nanophase effect. This suggests burial of continental crustal rocks to depths of at least 130-200 kilometers. Thus, α-PbO2-type TiO2 inclusions in garnet may be a useful P-T indicator in the diamond stability field. Furthermore, the possibility of finding α-PbO2-type TiO2 or even a higher-P polymorph (e.g., baddeleyite-structured TiO2) at impact sites of meteorite craters is increased, in view of the recent identification of post-stishovite (isostructure of rutile) SiO2 polymorphs in the meteorite Shergotty, and the alleged identification of α-PbO2-type TiO2 by Raman spectroscopy in shocked gneisses from the Ries Meteorite Crater, Germany.