Anomalous birefringent uvarovite (Uv) garnets from the Ural Mountains, Russia (SAR-1,
SAR-2, and SKR-1), Zermatt, Switzerland (STZ-1), Jacksonville, Tuolumne County, California (JTC-1), and Outokumpu, Finland (FIN-1) were analyzed using synchrotron high-resolution powder X-ray diffraction (HRPXRD) and electron microprobe analysis (EMPA) in order to understand the crystal chemistry and structure that causes the occurrence of optical anisotropy. HRPXRD detected the existence of fine scale intergrowths of two (SAR-2, STZ-1, and JTC-1), three (SAR-1), and up to four (FIN-1) additional cubic phases of Uv, with slight changes in the a unit-cell parameter representative of distinct chemical compositional phases obtained by EMP analyses. The a unit-cell parameter ranges from 11.91425(2) Å in SAR-1b to 12.05305(2) Å in JTC-4a and based on the garnet end-member composition, uvarovite ranges from Uv9Adr88Grs2 in JTC-4a to Uv71Grs24Sps1 in FIN-6a. Rietveld refinement X-ray diffraction traces show no deviation from cubic symmetry and reveal consistent peak profiles with the appearance of broadening and asymmetrical peak effects caused by the existence of additional microscopic cubic uvarovite phases intergrown together. These multiphase intergrowths contain strain derived from lattice mismatch of slightly different a unit-cell parameters against boundary contacts between phases and ultimately give rise to the observed birefringence in uvarovite, and possibly to other garnet end-member species.