Grossular Garnet

Grossular is the calcium aluminum end-member of the garnet group of silicate minerals (Ca₃Al₂(SiO₄)₃), occurring in a wide range of colors. Grossular garnet forms through contact and regional metamorphism of impure calcareous rocks. When limestone or dolostone containing clay minerals is heated by an intruding magma body, the calcium from carbonate, aluminum from clay, and silica recombine at temperatures between 500 and 700 degrees Celsius. This process, called skarn formation, creates grossular alongside other calcium silicate minerals like diopside, wollastonite, and vesuvianite. The specific color depends entirely on trace element substitutions during crystallization.

Chromium and vanadium produce the vivid green of tsavorite, which forms in unusual geological settings where these trace elements are concentrated. At Merelani in Tanzania, tsavorite crystallizes within graphite-bearing gneiss that was metamorphosed under medium-grade conditions around 600 degrees Celsius. The graphite acted as a reducing agent, keeping chromium and vanadium in the correct oxidation states to enter the garnet structure. Iron produces the cinnamon-orange of hessonite, while manganese creates pink to raspberry shades.

Colorless grossular, known as leuco garnet, forms in very pure calcium-aluminum systems with minimal trace element contamination. Massive green grossular, sometimes called Transvaal jade or hydrogrossular, contains hydroxyl groups substituting for some silicate tetrahedra, creating an opaque material dense enough to be carved. This variety forms at lower temperatures in serpentinite-hosted rodingite veins where water-rich fluids drive the reaction.

Grossular is identified by its isometric crystal habit (typically dodecahedral or trapezohedral forms), vitreous to resinous luster, and wide color range. The refractive index falls between 1.734 and 1.759, higher than pyrope but lower than andradite. Specific gravity of 3.57 to 3.73 distinguishes it from lighter garnets. Grossular is singly refractive (isotropic), which separates it from similar-looking doubly refractive gems. Green tsavorite can be distinguished from emerald by its lack of inclusions typical of beryl (three-phase inclusions, jardin), its higher refractive index, and its isotropic nature under a polariscope. Orange hessonite shows a distinctive roiled or heat-haze internal texture under magnification, sometimes called the "scotch in water" effect, which is diagnostic. Distinguish massive green grossular from nephrite jade by garnet's higher specific gravity (3.6 vs 2.95) and its granular rather than fibrous fracture.