Uvarovite

Uvarovite is the chromium-bearing, green end-member of the garnet group of silicate minerals (Ca₃Cr₂(SiO₄)₃). Uvarovite forms exclusively in chromium-rich geological environments, almost always in direct association with chromite ore bodies hosted within serpentinized ultramafic rocks. When chromite-bearing peridotite undergoes serpentinization through reaction with water at temperatures between 200 and 400 degrees Celsius, chromium is mobilized from the breakdown of chromite grain boundaries. This chromium combines with calcium from coexisting plagioclase or calcite and silica from the dissolving olivine and pyroxene to crystallize uvarovite in fractures and vugs within and around the chromite.

The crystals form as druzy coatings, rarely exceeding 2 to 3 millimeters in diameter. This persistent small crystal size is not accidental. Chromium has a strong preference for octahedral coordination in oxide minerals like chromite, and convincing it to enter the garnet structure requires specific fluid chemistry conditions that tend to be localized and short-lived. Once the chromium-bearing fluid is depleted or the temperature drops below about 200 degrees Celsius, growth stops. The few known occurrences of uvarovite crystals approaching 1 centimeter come from settings where chromium-rich fluids were sustained over longer periods, typically in large chromite ore bodies with extensive serpentinization.

The vivid green color is a direct consequence of Cr³⁺ ions occupying the octahedral aluminum site in the garnet structure. Each chromium ion absorbs light in the red and blue portions of the visible spectrum, transmitting a pure, saturated green that rivals emerald. Unlike most other green minerals, uvarovite's color is structural and permanent, never fading in light or heat.

Uvarovite is one of the easier garnets to identify because its combination of vivid emerald-green color and tiny crystal size on chromite matrix is essentially unique. The crystals form sharp dodecahedral or trapezohedral shapes, typically as glittering druzy crusts. The refractive index is approximately 1.87, and the specific gravity around 3.78. Under magnification, the consistent small crystal size and association with dark metallic chromite are diagnostic. Distinguish from chrome diopside by uvarovite's isometric crystal system (diopside is monoclinic with prismatic cleavage). Distinguish from emerald by the dodecahedral crystal habit (emerald forms hexagonal prisms), higher RI, and crusty growth habit. Uvarovite does not form the large transparent crystals seen in other green minerals. If offered a large faceted "uvarovite," it is almost certainly tsavorite, chrome tourmaline, or a synthetic. The mineral can be confused with bright green grossular (tsavorite) in rare cases where isolated small crystals occur, but uvarovite's higher RI and chromite association resolve any ambiguity.