Garnet

Garnet is not a single mineral but a group of closely related silicates that share the same crystal structure but differ in chemistry. They form under conditions of high temperature and pressure - primarily in metamorphic rocks (where existing rocks are transformed by heat and pressure) and in some igneous rocks.

Almandine (red) forms in metamorphic schists and gneisses at moderate to high pressures. Pyrope (deep red) forms at extreme pressures deep in the Earth's mantle and is brought to the surface in kimberlite pipes - the same geological structures that produce diamonds. This is why pyrope garnets are used by diamond prospectors as indicator minerals.

Tsavorite (green) forms in a narrow set of conditions where calcium, aluminum, vanadium, and chromium are all available - conditions met in the metamorphosed graphite-bearing rocks of East Africa. Its rarity, combined with a color that rivals emerald at a fraction of the price, makes it one of the most sought-after colored gemstones.

Garnets are most reliably identified by their crystal habit (dodecahedrons and trapezohedrons - 12 and 24-faced forms), high hardness (6.5-7.5), lack of cleavage, and high specific gravity. They show no cleavage and fracture conchoidally.

The most common confusion is between garnet and ruby (both can be deep red), but garnet is singly refractive while ruby is doubly refractive - a gemological test easily distinguishes them. Garnet also tends to be darker and less vivid than ruby. The wide range of garnet colors means specific variety identification often requires gemological testing.