How Do Garnets Form?

What is Garnet?

Did you know that garnet is a mineral group? Members of the garnet group all have similar mineral formulas: X₂Y₃(SiO₄)₃. Several different elements can fit into the X and Y spots in this formula, and each combination is a different garnet mineral.

The most common garnet minerals have aluminum in the “X” spot (pyrope, almandine, grossular, and spessartite) and/or calcium in the “Y” spot (andradite, grossular, and uvarovite). These species are also the most common for gem-quality garnets.

However, no actual garnet has a 100% pure chemical composition. Garnet gemstones are mixtures or blends, containing the chemistry of two or more garnet species.

How Do Garnets Form?

Regardless of the chemistry, all of the garnet group minerals form under similar geological conditions. Most of the time, garnet crystals form in metamorphic rocks. When sedimentary rocks like shale travel deep into the earth on a subducting plate, the heat and pressure alter the minerals in the rock. Garnets can form in these conditions, and some will even change chemical composition as they grow, reflecting the temperature and pressure at that…