Zirconium dioxide (ZrO2), sometimes called zirconia (not to be confused with zircon), is a white zirconium crystalline oxide. Its most natural form, with a monoclinic crystalline structure, is the mineral baddeleyite. A cubic zirconia dopant stabilized structured cubic zirconia is synthesized in different colors for use as a gemstone and a diamond simulant.
Zirconium dioxide is one of the most studied ceramic materials. ZrO2 adopts a monoclinic crystalline structure at room temperature and transitions towards tetragonal and cubic at higher temperatures. The change in volume caused by the tetragonal to cubic monoclinic structure transitions induces large stresses, which causes cracking during cooling of high temperatures.  When the zirconia is mixed with other oxides, the tetragonal and / or cubic phases are stabilized. Effective dopants include magnesium oxide (MgO), yttrium oxide (Y2O3, yttrium oxide), calcium oxide (CaO) and cerium (III) oxide (Ce2O3).
Zirconia is often more useful in its "stabilized" phase. During heating, the zirconia undergoes disturbing phase changes. By adding small percentages of yttria, these phase changes are eliminated and the resulting material has superior thermal, mechanical and electrical properties. In some cases, the tetragonal phase may be metastable. If sufficient amounts of the metastable quadratic phase are present, then an applied stress, amplified by the stress concentration at a crack tip, can cause the quadratic phase to convert to monoclinic, with the associated volume expansion. This phase transformation can then compress the crack, retarding growth, and improving fracture resistance. This mechanism is known as transformation quenching, and significantly extends the reliability and service life of products made with stabilized zirconia.
The band gap of ZrO2 depends on phase (cubic, tetragonal, monoclinic or amorphous) and preparation methods, with typical estimates of 5-7 eV (0.80-1.12 aJ). A particular case of zirconia is that of the tetragonal zirconia polycrystal, or TZP, which is indicative of polycrystalline zirconia composed solely of the metastable tetragonal phase.